This is the second CL in a line of two to implement PKU-based
WebAssembly code space write protection. The first CL added two
low-level PKU functions; this CL uses them to grant/withdraw writable
permissions, local to each thread that wants to modify the code space.

In particular, when {--wasm-memory-protection-keys} is enabled, we first
associate a memory protection key with all code pages, which by
default does not allow any write access. Then, before each location that
needs to modify the code space, we open
{NativeModuleModificationScope}s (which are already present for
mprotect-based write protection). When the PKU flag is given, this then
first tries to set permissions of a memory protection key (which is
fast), and otherwise when {--wasm-write-protect-code-memory} is enabled,
falls back to mprotect-based write protection (which is much more
expensive and also not thread-local, but for the whole process).

R=clemensb@chromium.org

Bug: v8:11714
Change-Id: I3527906a8d9f776ed44c8d5db52539e78e1c52fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2882800
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74501}