While compiling concurrently, we change the permissions of the page
containing the new code object to RWX, so the main thread can continue
executing a potential code in the same page.

If no thread is compiling the new code, we change the permissions
of all pages affected back to RX.

We also initialises code object page to immediately RWX by default.
Otherwise, a new code could be allocated in the same page, it will call
UnprotectAndRegister, and since write_unprotect_counter_ is now at
least 2, the code ignores the permission change. We then sigfault
when trying to run the new code.

Change-Id: Id18bcb9a44843b4ff747b1e4ac91913e80b74d80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3257606Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77827}