Add wrappers for arm64 atomics.

Add a _llsc suffix for the existing LL/SC-based implementations and add
trivial wrappers. This is in preparation for supporting LSE-based
atomic(9) implementations.

No functional change intended.

Reviewed by: andrew, kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation, Amazon (hardware)
Differential Revision: https://reviews.freebsd.org/D23323