cs: skip check for module as an extension

The "extension" module protocol predates the modern FFI and depends on
the C API. Since it's not supported on Racket CS, skip the check for
extension modules.

Skipping the check can reduce load time considerably. We should
consider depracting the extension protocol for traditional Racket.

pkgs/racket-doc/scribblings/reference/eval.scrbl

@@ -223,19 +223,28 @@ An @tech{extension-load handler} takes the same arguments as a
 (Mac OS).  The file is loaded using internal, OS-specific
 primitives. See @other-manual['(lib
 "scribblings/inside/inside.scrbl")] for more information on
-@tech{dynamic extensions}.}
+@tech{dynamic extensions}.
+
+Extensions are supported only when @racket[(system-type 'vm)] returns
+@racket['racket].}
 
 
 @defproc[(load-extension [file path-string?]) any]{
 
 Sets @racket[current-load-relative-directory] like @racket[load], and
-calls the @tech{extension-load handler} in tail position.}
+calls the @tech{extension-load handler} in tail position.
+
+Extensions are supported only when @racket[(system-type 'vm)] returns
+@racket['racket].}
 
 
 @defproc[(load-relative-extension [file path-string?]) any]{
 
 Like @racket[load-extension], but resolves @racket[file] using
-@racket[current-load-relative-directory] like @racket[load-relative].}
+@racket[current-load-relative-directory] like @racket[load-relative].
+
+Extensions are supported only when @racket[(system-type 'vm)] returns
+@racket['racket].}
 
 
 @defparam[current-load/use-compiled proc (path? (or/c #f
@@ -261,8 +270,8 @@ The protocol for a @tech{compiled-load handler} is the same as for the
    the default @tech{compiled-load handler} checks for a @filepath{.ss} file.}
 
  @item{The default @tech{compiled-load handler} checks for the opportunity
-   to load from @filepath{.zo} (bytecode) files and
+   to load from @filepath{.zo} (bytecode) files and, when @racket[(system-type 'vm)]
-   @filepath{.so} (native Unix), @filepath{.dll} (native Windows),
+   returns @racket['racket], for @filepath{.so} (native Unix), @filepath{.dll} (native Windows),
    or @filepath{.dylib} (native Mac OS) files.}
 
  @item{When the default @tech{compiled-load handler} needs to load from
@@ -290,7 +299,8 @@ order, and the subdirectories are checked in order within each root. A
 @filepath{.zo} version of the file (whose name is formed by passing
 @racket[_file] and @racket[#".zo"] to @racket[path-add-extension]) is
 loaded if it exists directly in one of the indicated subdirectories,
-or a @filepath{.so}/@filepath{.dll}/@filepath{.dylib} version of the
+or when @racket[(system-type 'vm)] returns
+@racket['racket], then a @filepath{.so}/@filepath{.dll}/@filepath{.dylib} version of the
 file is loaded if it exists within a @filepath{native} subdirectory of
 a @racket[use-compiled-file-paths] directory, in an even deeper
 subdirectory as named by @racket[system-library-subpath]. A compiled
@@ -299,7 +309,8 @@ file is loaded only if it checks out according to
 of @racket['modify-seconds], a compiled file is used only if its
 modification date is not older than the
 date for @racket[_file]. If both @filepath{.zo} and
-@filepath{.so}/@filepath{.dll}/@filepath{.dylib} files are available,
+@filepath{.so}/@filepath{.dll}/@filepath{.dylib} files are available
+when @racket[(system-type 'vm)] returns @racket['racket],
 the @filepath{.so}/@filepath{.dll}/@filepath{.dylib} file is used.  If
 @racket[_file] ends with @filepath{.rkt}, no such file exists, the
 handler's second argument is a symbol, and a @filepath{.ss} file

racket/collects/compiler/private/cm-minimal.rkt

@@ -840,9 +840,10 @@
 
 (define (get-compiled-time path->mode roots path)
   (define-values (dir name) (get-compilation-dir+name path #:modes (list (path->mode path)) #:roots roots))
-  (or (try-file-time (build-path dir "native" (system-library-subpath)
+  (or (and (eq? 'racket (system-type 'vm))
+           (try-file-time (build-path dir "native" (system-library-subpath)
                                       (path-add-extension name (system-type
-                                                           'so-suffix))))
+                                                                'so-suffix)))))
       (try-file-time (build-path dir (path-add-extension name #".zo")))))
 
 ;; Gets a multi-sha1 string that represents the compiled code
@@ -895,11 +896,12 @@
                                                                          (cadr roots)
                                                                          (car roots))))
   (let ([dep-path (build-path dir (path-add-extension name #".dep"))])
-    (or (try-file-sha1 (build-path dir "native" (system-library-subpath)
+    (or (and (eq? 'racket (system-type 'vm))
+             (try-file-sha1 (build-path dir "native" (system-library-subpath)
                                         (path-add-extension name (system-type
                                                                   'so-suffix)))
                             dep-path
-                       roots)
+                            roots))
         (try-file-sha1 (build-path dir (path-add-extension name #".zo"))
                        dep-path
                        roots)

racket/src/expander/boot/handler.rkt

@@ -111,6 +111,7 @@
                                     (date-of-1 alt-path))]
                    [path-d (or main-path-d alt-path-d)]
                    [get-so (lambda (file rep-sfx?)
+                             (and (eq? 'racket (system-type 'vm))
                                   (lambda (root-dir compiled-dir)
                                     (build-path (reroot base root-dir)
                                                 compiled-dir
@@ -120,7 +121,7 @@
                                                     (path-add-extension
                                                      file
                                                      dll-suffix)
-                                               file))))]
+                                                    file)))))]
                    [zo (lambda (root-dir compiled-dir)
                          (build-path (reroot base root-dir)
                                      compiled-dir
@@ -135,12 +136,14 @@
                    [try-alt? (and alt-file (or alt-path-d (not main-path-d)))]
                    [with-dir (lambda (t) (with-dir* base t))])
               (cond
-               [(and try-main?
+               [(and so
+                     try-main?
                      (date>=? modes roots so path-d))
                 => (lambda (so-d)
                      (parameterize ([current-module-declare-source #f])
                        (with-dir (lambda () ((current-load-extension) (car so-d) expect-module)))))]
-               [(and try-alt?
+               [(and alt-so
+                     try-alt?
                      (date>=? modes roots alt-so alt-path-d))
                 => (lambda (so-d)
                      (parameterize ([current-module-declare-source alt-path])