s390: add custom target and make path extension optional for .bss check

"chkbss-target" could be now used to override default target .bss check
is bound to. Objects to be checked are only path extended when needed.

Reviewed-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>