s390: add initial 64-bit restart PSW

To be able to start a kernel image loaded into memory with a
PSW restart, place a 64-bit restart PSW at 0x1a0 in absolute
lowcore.

Suggested-by: Dominik Klein <dominik.klein@linux.ibm.com>
Tested-by: Dominik Klein <dominik.klein@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>