[S390] smp: introduce LC_ORDER and simplify lowcore handling

Removes a couple of simple code duplications. But before I have to do
this again, just simplify it.

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