This is in preparation for supporting concurrent access in
String::SlowEquals, which will need to compare character buffers with
relaxed ordering.

Bug: v8:12007
Change-Id: Ie8ac62c15df48ebd605985c35b843b510c7ad167
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3313467
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78231}