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), this associates a key with each page once, and then changes
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) trying to allocate a protection key once per {WasmEngine} in x64
Linux systems, and
(2) adding a counter for recording the sucess/failure of that, to assess
the support for PKUs on the target machine.

The low-level PKU allocating functions should be moved into base/platform
long-term, but are inside wasm/ for this CL.

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

Bug: v8:11714
Change-Id: Ia4858970ced4d0b84cc8c2651e86dceb532c88a7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2850932
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74319}