Add atomic_testandset_acq_* on arm64

We only need to include sys/_atomic_subword.h on arm64 to provide
atomic_testandset_acq_long. Add an implementation in the arm64 atomic.h
based on the existing atomic_testandset macro.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33587