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.

Runtime internal usage of objc_classname_to_class() does not use the
fast path and does not count as a call into objc_classname_to_class().
The reason for this is that if the runtime calls
objc_classname_to_class(), it already has the lock and thus the
performance gain would be small, but it would waste memory.