s390/smp: remove redundant pcpu::lowcore member

Per-CPU pointer to lowcore is stored in global lowcore_ptr[]
array and duplicated in struct pcpu::lowcore member. This
update removes the redundancy.

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