With external code space and background compilation, external pointer
table entries are now allocated on background threads. For this to work
properly, the implementation must be atomic.

As atomic operations are not currently available in CSA, the fast path
in CSA::InitializeExternalPointerField has been removed for now.

Bug: v8:10391
Change-Id: I1119a9b5f97bc8d5f48de6872b62b9ddf001e9ce
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/+/3448381Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79037}