Rework the atomic code to reduce the repetition. This merges some of the
atomic functions where they are almost identical, or have acquire/release
semantics.

While here clean these function up. The cbnz instruction doesn't change
the condition flags so drop cc, however they should have memory added to the
clobber list.

Reviewed by: kib
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D4318