Add LJ_GC64 mode: 64 bit GC object references.

Actually NaN tagging with 47 bit pointers and 13+4 bit tags.

src/lj_state.c

@@ -207,7 +207,9 @@ LUA_API lua_State *lua_newstate(lua_Alloc f, void *ud)
   setnilV(registry(L));
   setnilV(&g->nilnode.val);
   setnilV(&g->nilnode.key);
+#if !LJ_GC64
   setmref(g->nilnode.freetop, &g->nilnode);
+#endif
   lj_buf_init(NULL, &g->tmpbuf);
   g->gc.state = GCSpause;
   setgcref(g->gc.root, obj2gco(L));