For shared strings, String::MakeThin is protected by using the map word
of the string being migrated as a spinlock.

Note that this CL does not make it safe yet to access character data
from multiple threads. The spinlock here only protects write-write races
in String::MakeThin.

For more information, see the following two design docs:

https://docs.google.com/document/d/1c5i8f2EfKIQygGZ23hNiGxouvRISjUMnJjNsOodj6z0/edit
https://docs.google.com/document/d/1Drzigf17t4ofy0evDmaIL5p0MDZuAl95c9fSeX-QjVg/edit

Bug: v8:12007
Change-Id: I9c47412c6ec7360a672b65a8576b4f6156ee5846
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3313429
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78257}