s390/smp,ptdump: add absolute lowcore markers

Add "Lowcore Area Start" and "Lowcore Area End" markers
that fence pages where absolute lowcore resides.

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