This CL does the following for x64:

- Add seq cst TSAN helpers.

- Refactors codegen's handling of TSAN helpers to also support
  seq cst accesses.

- Perform stores only once instead twice under TSAN, since
  duplicating stores is unsound. Previously this was "fine"
  because all duplicated stores were relaxed. SeqCst stores
  are used for synchronization, however, and duplicating them
  breaks the synchronization.

Bug: v8:7790, v8:11600, v8:11995
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Change-Id: I43071b0ed516cb0917a10f3b2b9861d74edca041
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3103308
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76612}