OBJC_CLASS as reference instead of objc_lookup_class for gnustep runtime

Hello David,

I decided to ask you before submitting patch here.

It’s about objc_lookup_class generation for every class member invocation,

ios runtime generates this case OBJC_CLASS_$_classname, the same behaviour I found

in objfw (CGObjCObjFW), also it took into account attribute(weak_import).

The question is following: gnustep runtime it’s none-fragile runtime as well as objcfw,

so is it worth to emit OBJC_CLASS_ reference in that place or I missed something?