Save currently executing lua_State in g->cur_L.

This is only a good approximation due to deficiencies in the design of
the Lua/C API. It indicates _some_ valid state that is/was executing.
Also reorder L->cframe stores to achieve a synchronously consistent state.

src/lj_dispatch.c

@@ -357,6 +357,7 @@ static void callhook(lua_State *L, int event, BCLine line)
     hook_enter(g);
     hookf(L, &ar);
     lua_assert(hook_active(g));
+    setgcref(g->cur_L, obj2gco(L));
     hook_leave(g);
   }
 }