Perform relaxed reads/writes on the builtin index.

Bug: v8:11478, v8:7790
Change-Id: I488fd30e6635f39bb46440e9d5230ab311825e7d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2716290Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73012}