Continuing the cleanups and using the tags rather than synchronized_
in the name of the accessors.

`map_word` (and its setter) can also be marked explicitly as relaxed to
show that they are atomic accessors.

Bug: v8:7790
Change-Id: I0725054ef37022c777f47660cb11cf839bb7f273
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2897094Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74605}