To be able to share external strings, we need to share the external
pointer table in sandbox builds.
To avoid branches at runtime all pointers for external strings are
stored in the shared external pointer table.

Bug: v8:12957
Change-Id: Iaa6be7839a2f5e50f80fd58c5b33fb9c6af61057
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695263Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81324}