Loading the value at the index for the futex wait should be protected by
the waiterlist mutex for both sync and async waits.

Bug: chromium:1194026
Change-Id: Ie9896cab6828763ebb963f5ad96f264d57c9377f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2796159
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73753}