Replace on-trace GC frame syncing with interpreter exit.

Need to sync GC objects to stack only during atomic GC phase.
Need to setup a proper frame structure only for calling finalizers.
Force an exit to the interpreter and let it handle the uncommon cases.
Finally solves the "NYI: gcstep sync with frames" issue.

src/lj_parse.c

@@ -1228,7 +1228,7 @@ static void fs_init(LexState *ls, FuncState *fs)
   fs->flags = 0;
   fs->framesize = 2;  /* Minimum frame size. */
   fs->kt = lj_tab_new(L, 0, 0);
-  /* Anchor table of constants and prototype (to avoid being collected). */
+  /* Anchor table of constants in stack to avoid being collected. */
   settabV(L, L->top, fs->kt);
   incr_top(L);
 }