[S390] kernel: always keep machine flags in lowcore

Eleminate the local variable machine_flags and always change machine
flags directly in the lowcore.
This avoids confusion about when and why the two variables have to be
synchronized.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>