misc4: Use BUILDFLAG for OS checking

Use // BUILDFLAG(IS_XXX) instead of // OS_XXX.

Generated by `os_buildflag_migration.py` (https://crrev.com/c/3311983)
with modifications.

R=thakis@chromium.org

Bug: 12340436
Change-Id: Ibcf60c672ab3167ddad14656bff27378e67f3975
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408124


Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Auto-Submit: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Owners-Override: Nico Weber <thakis@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962181}