Fix -Winvalid-constexpr instances.

AslrMask() is marked as constexpr but AslrMask(uintptr_t bits) is not.
This fixes it by marking AslrMask() as PAGE_ALLOCATOR_CONSTANTS_DECLARE_CONSTEXPR
which is __attribute__((const)). Same for ASLROffset().

Bug: 330775073
Change-Id: Iced82a104d76cee320dcbaa13694920aeb5177e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5387606


Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Zequan Wu <zequanwu@google.com>
Cr-Commit-Position: refs/heads/main@{#1277222}