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_gc.h

@@ -47,7 +47,7 @@ LJ_FUNC void lj_gc_freeall(global_State *g);
 LJ_FUNCA int LJ_FASTCALL lj_gc_step(lua_State *L);
 LJ_FUNCA void LJ_FASTCALL lj_gc_step_fixtop(lua_State *L);
 #if LJ_HASJIT
-LJ_FUNC void LJ_FASTCALL lj_gc_step_jit(lua_State *L, MSize steps);
+LJ_FUNC int LJ_FASTCALL lj_gc_step_jit(lua_State *L, MSize steps);
 #endif
 LJ_FUNC void lj_gc_fullgc(lua_State *L);