Introduces an AtomicUint64 type and a seperate AtomicLoad64 due to the
different types returned by loading 64-bit atomic values on 32-bit vs
64-bit architectures.

BUG=v8:6949,v8:11074

Change-Id: I95de994df9639847cd6b5fd56ea2a6585189ed3a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2529455
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71110}