The JSArrayBuffer::extension-field might not be aligned with pointer
compression enabled. However on AArch64 pointers need to be aligned if
you perform atomic operations on them. Therefore split extension into
two 32-bit words that each get updated atomically. There is no ABA
problem here since the extension field only transitions from
NULL --> value --> NULL. After Detach(), Attach() isn't invoked anymore.

Bug: v8:10064
Change-Id: If987ed51f0528ca7313980f3d36ffca300b75fdc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2071256
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66457}