- Add Implementation for I64Atomic{Load, Store, Exchange,
CompareExchange} for supported MemTypes/Representations
 - Refactoring to simplify instruction selection
 - Enable tests for ARM64

Bug: v8:6532
Change-Id: I4c4a65fd3bbdc6955eda29d7e08d6eef29c55628
Reviewed-on: https://chromium-review.googlesource.com/1003225Reviewed-by: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52598}