Add LJ_FR2 mode: Two-slot frame info.

src/lj_lib.h

@@ -47,8 +47,13 @@ LJ_FUNC GCtab *lj_lib_checktabornil(lua_State *L, int narg);
 LJ_FUNC int lj_lib_checkopt(lua_State *L, int narg, int def, const char *lst);
 
 /* Avoid including lj_frame.h. */
+#if LJ_FR2
+#define lj_lib_upvalue(L, n) \
+  (&gcref((L->base-2)->gcr)->fn.c.upvalue[(n)-1])
+#else
 #define lj_lib_upvalue(L, n) \
   (&gcref((L->base-1)->fr.func)->fn.c.upvalue[(n)-1])
+#endif
 
 #if LJ_TARGET_WINDOWS
 #define lj_lib_checkfpu(L) \