Instead of implementing our own shared mutex, use std::shared_mutex,
which does not have the problem of deadlocking when interrupted by
signals (see https://crbug.com/v8/12037).
This is on Mac only, for now. If this fixes the regressions, we can
switch all systems to std::shared_mutex.

R=ishell@chromium.org
CC=dmercadier@chromium.org

Bug: v8:12037, v8:13256, chromium:1358856
Change-Id: Ie4be7cc5431905ca1e4f74809168eb6a9f584d28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3870465
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82933}