atomic: Add atomic_cmpset_masked to powerpc and use it

Summary:
This is a more optimal way of doing atomic_compset_masked() than the
fallback in sys/_atomic_subword.h. There's also an override for
_atomic_fcmpset_masked_word(), which may or may not be necessary, and is
unused for powerpc.

Reviewed by: kevans, kib
Differential Revision: https://reviews.freebsd.org/D22359