Lock mutex for shared isolate in global safepoints, such that e.g. the
StringTable can use isolate->heap()->safepoint()->AssertActive() even
for shared isolates.

Bug: v8:11708, v8:12749
Change-Id: I8d99203581dfa2d7225846e19fa981300f88589e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3563138Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79715}