arm64: atomics: implement native {relaxed, acquire, release} atomics

Commit 654672d4

 ("locking/atomics: Add _{acquire|release|relaxed}()
variants of some atomic operation") introduced a relaxed atomic API to
Linux that maps nicely onto the arm64 memory model, including the new
ARMv8.1 atomic instructions.

This patch hooks up the API to our relaxed atomic instructions, rather
than have them all expand to the full-barrier variants as they do
currently.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>