fix native-stacktrace interaction with thread start

Repairs 4d3852ae69.

racket/src/racket/src/jitcommon.c

@@ -3663,7 +3663,8 @@ static int more_common0(mz_jit_state *jitter, void *_data)
     jit_ret();
     CHECK_LIMIT();
 
-    scheme_jit_register_sub_func(jitter, sjc.thread_start_child_code, scheme_eof);
+    /* No scheme_jit_register_sub_func, because we don't want to try to
+       traverse past this frame for a native stack trace. */
   }
 
   return 1;

racket/src/racket/src/jitstack.c

@@ -719,6 +719,7 @@ void *scheme_module_start_start(struct Start_Module_Args *a, Scheme_Object *name
 }
 
 void scheme_thread_start_child(Scheme_Thread *child, Scheme_Object *child_thunk)
+  XFORM_SKIP_PROC
 {
   Thread_Start_Child_Proc proc = (Thread_Start_Child_Proc)sjc.thread_start_child_code;
   if (proc)

racket/src/racket/src/thread.c

@@ -3168,6 +3168,7 @@ static void start_child(Scheme_Thread * volatile child,
 }
 
 void scheme_do_thread_start_child(Scheme_Thread *child, Scheme_Object *child_eval)
+  XFORM_SKIP_PROC
 {
   return start_child(child, child_eval);
 }