s390/defconfigs: set CONFIG_INIT_STACK_NONE=y

Set CONFIG_INIT_STACK_NONE=y in defconfigs to avoid the extra overhead of
initializing all stack variables by default. Users who want to have that
must change the configuration on their own.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>