AtomicWord will either alias Atomic32 or Atomic64, depending on the
platform. By slightly changing the definition to encode this directly
instead of relying on intptr_t, we can get rid of a number of
compatibility helpers that cast between pointers to equally sized
atomics.

R=mlippautz@chromium.org

Bug: v8:12425
Change-Id: I04e8433cba5af8cf398d75d7832b84680109cf8b
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3586988Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80017}