The previous AtomicElement wrapper fundamentally relied on
reinterpret_casting a heap address to an instance of a C++
object, which is an invalid cast. This patch replaces that
pattern with an ObjectSlot-based alternative that does not
rely on UB.

Bug: v8:3770
Change-Id: I62fb3c7589ac59e9e18139b525174de77e0e2149
Reviewed-on: https://chromium-review.googlesource.com/c/1309297
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57197}