Since those are accessed from background threads during marking, they
should generally be loaded and stored using atomic operations. Further,
when an external pointer slot is initialized, the handle should be
stored using release semantics to prevent reordering of the store into
the pointer table after the store of the handle to the object.

Bug: v8:10391, v8:13156
Change-Id: I5c33b4e791482f84e2770cd047a11f5762a0aa65
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3812035Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82280}