To enforce W^X for the WebAssembly code space, we want to explore using
Intel memory protection keys for userspace, also known as MPK, PKEYs, or
PKU. Instead of flipping page protection flags with mprotect (which
incurs a high syscall overhead; and which switches flags for the whole
process), with PKU we associate a key with each page once and then
change the permissions of that key with a fast thread-local register
write. That is, this gives both finger-grained permissions (per-thread)
and more performance.

This CL is starts experimenting with PKUs by
(1) adding a flag to turn on prototype PKU support; and if set to true
(2) allocates a protection key once per {WasmCodeManager} in x64 Linux
systems.

This is a partial reland of https://crrev.com/c/2850932, which was
reverted due to an added histogram failing Chromium integration.
Since the histogram (to record PKU support) is independent of the
functionality in this CL, we split it out into its own CL (to come).

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

Bug: v8:11714
Change-Id: I67c8679495c55fa51da8243582963649abde660b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2878738
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74435}