Provide a single implementation for each of the arm64 atomic(9) ops.

Parameterize the macros by type width as well as acq/rel semantics.
This makes modifying the implementations much less tedious and
error-prone and makes it easier to support alternate LSE-based
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/D23322