The {WasmCodeManager::SetThreadWritable} method would return true if
called in a nested scope, even if PKU is not available. The caller
cannot tell then whether permission switching happened or not.

This CL refactors the code to do an explicit check for PKU support, and
removes the boolean return value from {SetThreadWritable}.

R=jkummerow@chromium.org

Bug: v8:11959, v8:11974
Change-Id: I2d45f1fa240305c6f92f63cdf190131d637bfe95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3021383
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75699}