Remove class_isKindOfClass().

This was never used and is not part of the Apple runtime either.
apple-forwarding-arm64.S: Fix two bugs.

The offset for the stored x0 was wrong so that comparing the result of
the call to -[forwardingTargetForSelector:] with self did not work. The
other bug was that x19 was not restored before jumping to
of_method_not_found.