Convert to UNSAFE_TODO in partition_alloc in mac_linux_pkey_asm

This is a reland of commit 28c7cb6b99dffd261c254343f151aa2021602866
https://chromium-review.googlesource.com/c/chromium/src/+/7157555

The diff was adding "Linux ChromiumOS MSan Builder" to the script.  Then
I splat the patch to ease review and ensure most can ship without
revert.  It has been split in 12 patches.

Original change's description:
-----------------------------
Convert to UNSAFE_TODO in partition_alloc

This is an automated #cleanup patch using the [Script] below.

We are migrating from coarse-grained file-level suppression (#pragma
allow_unsafe_buffers) to granular, expression-level markers
(UNSAFE_TODO()). The pragma disables safety checks for an entire file,
whereas UNSAFE_TODO() isolates specific potentially unsafe operations,
allowing the rest of the file to be enforced as safe.

This CL was uploaded by git cl split.

Script: https://docs.google.com/document/d/1ORQGBNn2R-CEvNbDTjRd-GrOBOFlCxIHdcvSUA_EhR4/edit?usp=sharing
AX-Relnotes: N/A
Cleanup: This is an automated #cleanup.
Bug: 409340989
Cq-Include-Trybots: luci.chromium.try:linux_chromium_msan_rel_ng,linux_chromium_chromeos_msan_rel_ng,ios-asan
Change-Id: I21b32a3b4e537d4f7990d35972f006db7f3f40db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7198002
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1551937}