s390/build: remove obsolete -mkernel-backchain flag

-mkernel-backchain cc flag is obsolete since quite a while, and not
present in minimal (supported by s390) gcc version 4.3. Removing it.

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>