Add a fast path for objc_classname_to_class().

This should improve the performance for the GCC ABI, as
objc_classname_to_class() is used for all sorts of class lookups, e.g.
objc_lookup_class().

As this performance improvement needs RAM, it is only used after 128
calls into objc_classname_to_class(), so that if the ObjFW ABI is used
and the user does not call into objc_getClass() or similar in a loop, no
memory is wasted.