What we need is a multiple readers single writer (MRSW) lock. The
main thread is the only one that is going to be writing, while the
readers might be either the main thread or background threads.

The shared_mutex is in the isolate itself, so that different isolates
will not block each other.

Bug: v8:7790
Change-Id: Idd6bb1826bd0cc6279df1c0694a84e00d53a7eae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2241513
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68639}