use "_LOCK" prefix for Windows cross-build

Use "_LOCK" instead of ".LOCK" when the cross
platform is Windows, not just when the current
platform is Windows.

Merge to v6.3

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

@@ -3,7 +3,8 @@
           (for-label framework/preferences
                      racket/runtime-path
                      launcher/launcher
-                     setup/dirs))
+                     setup/dirs
+                     setup/cross-system))
 
 @(define file-eval (make-base-eval))
 @(interaction-eval #:eval file-eval (begin (require racket/file) (define filename (make-temporary-file))))
@@ -1402,7 +1403,8 @@ in the sense of @racket[port-try-file-lock?].
                                  [name path-element?]) 
             path?])]{
 
-Creates a lock filename by prepending @racket["_LOCK"] on Windows or
+Creates a lock filename by prepending @racket["_LOCK"] on Windows
+(i.e., when @racket[cross-system-type] reports @racket['windows]) or
 @racket[".LOCK"] on other platforms to the file portion of the path.
 
 @examples[

racket/collects/racket/file.rkt

@@ -1,6 +1,7 @@
 #lang racket/base
 (require "path.rkt"
          setup/dirs
+         setup/cross-system
          (for-syntax racket/base
                      setup/path-to-relative))
 
@@ -278,7 +279,7 @@
 (define (make-pathless-lock-file-name name)
   (bytes->path-element
    (bytes-append
-    (if (eq? 'windows (system-type))
+    (if (eq? 'windows (cross-system-type))
         #"_"
         #".")
     #"LOCK"