This cl makes IsLockFree return true for 8 bytes on x64 platforms.
The standard is unfortunately a bit vague on what exactly 'lock free' means.
As a result, we err on the side of caution. We can revisit this, but first
we need the specification to nail down exactly what 'lock free' in this
context.

Bug: v8:8100
Change-Id: I0a6099c6cb95a5581f3e71d0267857b88b4a2f0a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735592
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63099}