expander: fix registry-lock retry path

Provoking the retry path is extremely difficult, so it's unlikely to
have caused any problem so far.

racket/src/expander/namespace/registry.rkt

@@ -28,7 +28,7 @@
             void]
            [else
             ;; CAS failed; take it from the top
-            (loop)]))
+            (lambda () (loop))]))
         (lambda ()
           (semaphore-post sema))))]
      [(eq? (current-thread) (cdr v))

racket/src/racket/src/startup.inc

@@ -12064,7 +12064,7 @@ static const char *startup_source =
 "(lambda()"
 "(if(box-cas! lock-box_0 v_31 lock_0)"
 "(let-values()(begin(proc_2) void))"
-"(let-values()(loop_74))))"
+"(let-values()(lambda()(loop_74)))))"
 "(lambda()(semaphore-post sema_0)))))))"
 "(if(eq?(current-thread)(cdr v_31))"
 "(let-values()(proc_2))"