Just as OS::Abort, OS::ExitProcess never returns. This is a minor
cleanup which is not expected to reduce binary size considerably, since
we do not call this function often. It is just nice to annotate it
correctly.
On windows, the {TerminateProcess} call is known to not return if
called on the current process. Add an {UNREACHABLE} to enforce this.

R=mlippautz@chromium.org

Bug: v8:11074
Change-Id: I5c079fc459685c65f932404ce536ea28ad188073
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2536634Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71210}