bc: fix GC cooperation on internal setjmp

The direct use of `__gc_var_stack__[1]` is wrong if xform has not
generated a function-call wrapper than sets that array element.

racket/collects/compiler/private/xform.rkt

@@ -3877,6 +3877,14 @@
                                                                 "array access"))
                                                        memcpy?)])
                      (loop (cdr e-) (cons v result) live-vars #t)))]
+                [(eq? '|XFORM_CURRENT_COUNT| (tok-n (car e-)))
+                 (loop (cdr e-)
+                       (cons (make-tok (string->symbol (format "~a_COUNT" (live-var-info-tag live-vars)))
+                                       (tok-line (car e-))
+                                       (tok-file (car e-)))
+                             result)
+                       live-vars
+                       #t)]
                 [(and (assq (tok-n (car e-)) vars)
                       (not (assq (tok-n (car e-)) (live-var-info-vars live-vars))))
                  ;; Add a live variable:

racket/src/racket/include/scheme.h

@@ -1701,7 +1701,7 @@ MZ_EXTERN void scheme_jit_setjmp_prepare(mz_jit_jmp_buf b);
                                GC_variable_stack = (void **)(void*)(b).gcvs, \
                                scheme_jit_longjmp((b).jb, v))
 # define scheme_setjmp(b)     ((b).gcvs = (intptr_t)__gc_var_stack__, \
-                               (b).gcvs_cnt = (intptr_t)(__gc_var_stack__[1]), \
+                               (b).gcvs_cnt = XFORM_CURRENT_COUNT, \
                                scheme_jit_setjmp((b).jb))
 #else
 # define scheme_longjmp(b, v) scheme_jit_longjmp(b, v)