The Arm atomic operations used an input register as a temporary,
corrupting it for future uses. Fix this by adding another temporary,
the impact of which is partly reduced by removing the "unique"
requirement on the address base register.

Bug: 
Change-Id: I99a7bc3c14100d64cb7478e2053cf83ab6dcea50
Reviewed-on: https://chromium-review.googlesource.com/795953
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49785}