cs: allow unsafe finalizers on managed objects

Change the regular weak reference in a custodian, which allows
`will-executor`-based finalization, to a "late" weakk reference, which
allows both `will-executor` and `register-finalizer` finalization.

Closes #3352

pkgs/racket-test/tests/racket/custodian-finalize-help.rkt

@@ -0,0 +1,25 @@
+#lang racket/base
+(require ffi/unsafe/custodian)
+
+;; This module is run by "custodian-finalize.rkt"
+
+(define (log-info x)
+  (log-message (current-logger) 'info 'finalize x))
+
+(define (finalize x)
+  (log-info "finalizing"))
+
+(define the-box
+  (let ([v (string-copy "a value")])
+    (register-finalizer-and-custodian-shutdown v finalize #:at-exit? #t)
+    (log-info "created")
+    (box v)))
+
+(module* check-gc #f
+  (set-box! the-box #f)
+  (collect-garbage 'major)
+  (sync (system-idle-evt))
+  (log-info "exiting"))
+
+(module* check-exit #f
+  (log-info "exiting"))

pkgs/racket-test/tests/racket/custodian-finalize.rkt

@@ -0,0 +1,30 @@
+#lang racket/base
+(require compiler/find-exe
+         racket/system)
+
+(define racket (find-exe))
+
+(define result 0)
+
+(define (run/get-stderr . args)
+  (define o (open-output-bytes))
+  (parameterize ([current-error-port o])
+    (apply system* racket args))
+  (get-output-bytes o))
+
+(define (check expect got)
+  (unless (equal? expect got)
+    (eprintf "Expected ~s,\n     got ~s\n" expect got)
+    (set! result 1)))
+
+(check #"finalize: created\nfinalize: exiting\nfinalize: finalizing\n"
+       (run/get-stderr "-W" "info@finalize"
+                       "-e"
+                       "(dynamic-require '(submod \"custodian-finalize-help.rkt\" check-exit) #f)"))
+
+(check #"finalize: created\nfinalize: finalizing\nfinalize: exiting\n"
+       (run/get-stderr "-W" "info@finalize"
+                       "-e"
+                       "(dynamic-require '(submod \"custodian-finalize-help.rkt\" check-gc) #f)"))
+
+(exit result)

racket/src/cs/schemified/thread.scm

@@ -5679,7 +5679,7 @@
              #f
              (let ((we_0
                     (if (not weak?7_0)
-                      (|#%app| host:make-will-executor void)
+                      (|#%app| host:make-late-will-executor void)
                       #f)))
                (begin
                  (let ((app_0 (custodian-children cust12_0)))

racket/src/thread/custodian.rkt

@@ -122,7 +122,7 @@
      [(custodian-shut-down? cust) #f]
      [else
       (define we (and (not weak?)
-                      (host:make-will-executor void)))
+                      (host:make-late-will-executor void)))
       (hash-set! (custodian-children cust)
                  obj
                  (cond