Currently, we have two different classes for switching the WebAssembly
generated code space to writable (e.g., before patching jump tables, or
when adding or removing code): `CodeSpaceWriteScope` (with the macro
`CODE_SPACE_WRITE_SCOPE`) and `NativeModuleModificationScope`.
The former was introduced for Apple Silicon ARM64 hardware ("Apple M1"),
which uses `MAP_JIT` + `pthread_jit_write_protect_np()` to change memory
permissions. The latter uses either Intel PKU (aka. memory protection
keys) to switch permissions (fast and thread-local, like on M1), and
alternatively `mprotect()`, on systems that do not have PKU support.

Since both classes serve the same purpose just with different
implementations on different platforms, we want to merge them in
follow-up CLs. As a first step, here we align all uses of
`CODE_SPACE_WRITE_SCOPE` with existing `NativeModuleModificationScope`s.
The two had diverged due to optimization work, where we moved
`NativeModuleModificationScope`s around (pulling them out of loops and
across function boundaries) to lower the amount of mprotect switches.

This should have none, or at best a very small positive performance
impact on Apple M1, since we now also switch less often (even though
switching should be very cheap). In terms of security, this in theory
makes the code space writable for longer time spans, but this is
probably not a large effect because
(1) we often moved the scope outside of loops, where it was open for
every iteration anyway, or
(2) in some cases a CODE_SPACE_WRITE_SCOPE was open somewhere on the
call stack already.

R=jkummerow@chromium.org
CC=clemensb@chromium.org

Bug: v8:11714
Change-Id: Id8744429e1183e118ab5e078750d294a99c9dce0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2968946Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Cr-Commit-Position: refs/heads/master@{#75230}