s390/setup: rearrange absolute lowcore initialization

Make the absolute lowcore assignments immediately follow
the boot CPU lowcore same member assignments. This way
readability improves when reading from up to down, with
no out of order mcck stack allocation in-between.

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