We had one use of OS_CHROMEOS in mksnapshot. OS_CHROMEOS is defined if
gn's `is_chromeos` is true, which checks `current_os`. `current_os !=
target_os` can happen if we're building with a non-default toolchain,
which happens often on CrOS, since `mksnapshot` is a host binary.

Tested by manually verifying that .text.hot.embedded now shows up on
arm32/aarch64 builds of embedded.S.

Bug: v8:9103
Change-Id: I038b56f4c18c7dd9a651ce676a977697dad14ae6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1573041
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60925}