Add pointer-sized field extension to the JSArrayBuffer class. Only
reserve space for this field when feature is enabled for now.

Bug: v8:10064
Change-Id: Idb6fdcdce2a048e6aed9a892bc46ce029e1119f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1956166Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65471}