Change TestPlatform's old_platform_ field to an atomic, so that its
write in the constructor and subsequent reads (that can be cross thread)
are race-free.

Hopefully this fixes the TSAN flakes we've been seeing.

Bug: v8:9054
Change-Id: Ib2b6edd21508ffa18debd6326dd7a100266125f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3506505
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79385}