cgc: repair for fsemaphore-wait

racket/src/racket/src/fun.c

@@ -9011,10 +9011,6 @@ Scheme_Lightweight_Continuation *scheme_capture_lightweight_continuation(Scheme_
   Scheme_Lightweight_Continuation *lw;
   void *stack;
 
-#ifndef MZ_PRECISE_GC
-  return NULL;
-#endif
-
   storage[1] = p;
 
   lw = MALLOC_ONE_RT(Scheme_Lightweight_Continuation);

racket/src/racket/src/future.c

@@ -2461,6 +2461,11 @@ static int capture_future_continuation(Scheme_Future_State *fs, future_t *ft, vo
   Scheme_Object **arg_S;
   void **stack;
 
+#ifndef MZ_PRECISE_GC
+  if (scheme_use_rtcall)
+    return 0;
+#endif
+
   storage[2] = ft;
 
   if (for_overflow) {