[S390] Use copy_to_absolute_zero() instead of "stura/sturg"

Use the new copy_to_absolute_zero() function instead of manual "stura"
and "sturg" to make the code shorter and more readable.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>