... for V8 code space. The feature is currently disabled.

In order to use fast W^X permission switching we must allocate
executable pages with readable writable executable permissions (RWX).
However, MacOS on ARM64 ("Apple M1"/Apple Silicon) prohibits further
permission changing of RWX memory pages. This means that the code page
headers must be allocated with RWX permissions too because otherwise
it wouldn't be possible to allocate a large code page over the freed
regular code page and vice versa.

When enabled, the new machinery works as follows:

1) when memory region is reserved for allocating executable pages, the
   whole region is committed with RWX permissions and then decommitted,
2) since reconfiguration of RWX page permissions is not allowed on
   MacOS on ARM64 ("Apple M1"/Apple Silicon), there must be no attempts
   to change them,
3) the request to set RWX permissions in the executable page region
   just recommits the pages without changing permissions (see (1), they
   were already allocated as RWX and then discarded),
4) in order to make executable pages inaccessible one must use
   OS::DiscardSystemPages() instead of OS::DecommitPages() or
   setting permissions to kNoAccess because the latter two are not
   allowed by the MacOS (see (2)).
5) since code space page headers are allocated as RWX pages it's also
   necessary to switch between W^X modes when updating the data in the
   page headers (i.e. when marking, updating stats, wiring pages in
   lists, etc.). The new CodePageHeaderModificationScope class is used
   in the respective places. On unrelated configurations it's a no-op.

The fast permission switching can't be used for V8 configuration with
enabled pointer compression and disabled external code space because
a) the pointer compression cage has to be reserved with MAP_JIT flag
   which is too expensive,
b) in case of shared pointer compression cage if the code range will
   be deleted while the cage is still alive then attempt to configure
   permissions of pages that were previously set to RWX will fail.

This also CL extends the unmapper unit tests with permissions tracking
for discarded pages.

Bug: v8:12797
Change-Id: Idb28cbc481306477589eee9962d2e75167d87c61
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3579303Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80238}