PR 34052 kill: Terminate inferior with non-zero exit code on Windows

Don't terminate the inferior with exit code 0 (a 'success' exit
code). Rather, use the standard exit code 137 as with SIGKILL on
Linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34052
Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>