arm64: cmpxchg: implement cmpxchg64_relaxed

This patch introduces cmpxchg64_relaxed for arm64 using the existing
cmpxchg_local macro, which performs a cmpxchg operation (up to 64 bits)
without barrier semantics.

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