s390/head.s: use zero as address for stfl

The architecture suggests to use address 0 as parameter for stfl,
to allow for future extensions. Using __LC_STFL_FAC_LIST (0x200)
shows which address is used, but might be not future proof.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>