Apple's upcoming arm64 devices will prevent rwx access to memory,
but in turn provide a new per-thread way to switch between write
and execute permissions. This patch puts that system to use for
the WebAssembly subsystem.
The approach relies on CodeSpaceWriteScope objects for now. That
isn't optimal for background threads (which could stay in "write"
mode permanently instead of toggling), but its simplicity makes
it a good first step.

Background:
https://developer.apple.com/documentation/apple_silicon/porting_just-in-time_compilers_to_apple_silicon

Bug: chromium:1117591
Change-Id: I3b60f0efd34c0fed924dfc71ee2c7805801c5d42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2378307
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69791}