This is a reland of 296fa964. The fix is
to dynamically check for alignment instead of relying on
{alignof(ElementType)}. I updated the comment to state that independent
of pointer compression we do not guarantee the alignment that the
compiler assumes (hence we rely on undefined behaviour here).

Original change's description:
> [elements] Avoid racy data reads/writes
>
> Instead of annotating those racy reads / writes to be ignore by TSan,
> just use relaxed atomics. This makes us not rely on undefined behaviour,
> and is unlikely to introduce noticeable overhead.
>
> This removes the only uses of TSAN_ANNOTATE_IGNORE_WRITES_BEGIN and
> friends, which allows us to remove the whole tsan.h header.
>
> R=ulan@chromium.org
> CC=​mlippautz@chromium.org
>
> Bug: v8:11704
> Change-Id: Ie6694c0ae5b40856b56fb97253ce626ec1f4c263
> Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
> Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2859957
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74321}

Bug: v8:11704
Change-Id: If75674785ca776dac06ed821f0032f865793dd77
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_odroid_arm_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2867479Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74402}