Once we start protecting assembler buffers via PKU, the background
compile threads need to enable write permissions during compilation in
order to access them.
This CL adds that, with a little trick to allow passing {nullptr} to the
{CodeSpaceWriteScope} for configurations where we do not actually use
the provided {NativeModule}. This is needed because the background
compile threads do not keep the {NativeModule} alive during compilation,
so they also should not pass the potentially dangling pointer to the
{CodeSpaceWriteScope}.

R=thibaudm@chromium.org

Bug: v8:12809
Change-Id: I1a1d4d41239a6b4497f8a12fb252d7da76e72e1a
Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3593117Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80152}