partition_alloc: Move LINUX_NAME_REGION define to its own header

It is not really a constant, and it is only used in a few places, so
move it out of page_allocator_constants.h. This avoids transitively
including <sys/prctl> in 70% of all translation units.

Change-Id: I9bfa73ccf2e61752fd0aa08655e03d3b475e7ad9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8300913
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1692596}
NOKEYCHECK=True
GitOrigin-RevId: 62584b68fa64cab8301ef848b86661065f7ff497