misc5: Use BUILDFLAG for OS checking

Replace malformed defined(OS_XXX with BUILDFLAG(IS_XXX).

Bug: 12340436
Test: No functionality change
Change-Id: I62b4fae4af7f105d8d1e1651078c315e1bc4df0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408361


Auto-Submit: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962220}