s390: disable postinit-readonly for now

This is a temporary fix to let lkdtm run again on s390, though it'll
still fail the ro_after_init tests. Until rodata and ro_after_init
sections can be split on s390, disable special handling of ro_after_init.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>