pthread_rwlock_t can deadlock on Mac if signals are sent to the process
in the wrong moment. Since we use processes e.g. for sampling profiling
(in both d8 and in Chrome), we hence cannot safely use pthread_rwlock_t
on Mac. Instead, fall back to a non-shared pthread_mutex_t.

Interestingly, this shows no measurable performance impact in Wasm
compilation on my MBP.

R=mlippautz@chromium.org

Bug: v8:11399
Change-Id: Ie8bfd5288bba8c4f3315ee4502b39b59d39c9bbd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3060480Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76015}