s390/bios: build with -fdelete-null-pointer-checks

Starting with version 4.9, GCC assumes it can't safely dereference null
pointers, and uses this for some optimizations. On s390, the lowcore
memory is located at address 0, so this assumption is wrong and breaks
the s390-ccw firmware. Pass -fdelete-null-pointer-checks to avoid that.

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <1434363843-14576-1-git-send-email-aurelien@aurel32.net>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>