[S390] bss section clearing.

Clear only memory from __bss_start to __bss_stop when clearing the bss
section. Not until _end, which currently happens to be the same.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>