s390/head: get rid of 31 bit leftovers

Get rid of old 31 bit leftovers within ipl code:

- convert everything to pc relative code
- use 64 bit addressing mode as early as possible
- use 64 bit arithmetics wherever possible

This way the code doesn't look as odd as before anymore.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>