s390: avoid double handling of "noexec" option

"noexec" option is already parsed during startup and its value is
exposed via noexec_disabled variable. Simply reuse that value during
machine facilities detection.

Suggested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>