atomic: Add atomic_cmpset_masked to powerpc and use it

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.

(cherry picked from commit d0bdb11139424b9dcfe3b73cd5a003055382de03)