We sometimes hit the DCHECK in the wasm code manager:
  DCHECK_IMPLIES(writable, !MemoryProtectionKeyWritable());

This is because we spawn new threads while having a
{CodeSpaceWriteScope} open. In the case of PKU, this changes the PKRU
register to allow writes to the code space, and the value of that
register is inherited by any new thread. If this thread then tries to
switch to writable code spaces, it hits the DCHECK. It would hit a
similar DCHECK when trying to execute code.

We fix this issue by temporarily resetting the PKRU register to
non-writable while we call the {NotifyConcurrencyIncrease} method. This
is not a very robust solution, as any new call that potentially happens
inside a {CodeSpaceWriteScope} needs to do the same, but refactoring the
code to avoid spawning new threads while being in writable state would
be a lot of work with other downsides.

R=jkummerow@chromium.org

Bug: v8:13075
Change-Id: Ibc7270aa597902dc6d9649cb6bcdfce8b1a9bafc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3762579
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81729}