s390/build: add *.o.chkbss files to targets list

Adding *.o.chkbss files to targets list makes sure that the kbuild is
aware of them and removes them during make clean.

Also remove *.o.chkbss file before an actual check, to avoid having
stale *.o.chkbss file left even if the check is failed.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>