[gin] Pkey mprotect thread isolation data

Assign the thread isolation pkey to the global struct that's holding the
thread isolation data.

This makes it write-protected, preventing possible bypass of the thread
isolation feature in case of a presence of a write gadget (on platforms
where the isolation feature is supported).

Bug: 493206872
Change-Id: Iab8bf4da852a7c64977be9ffe0eddc6453c628a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7748632
Reviewed-by: Stephen Röttger <sroettger@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1629704}
NOKEYCHECK=True
GitOrigin-RevId: 638f2dda7d7096fc45b53df3c9036128010d1a26