s390: add alignment hints to vector load and store

The z14 introduced alignment hints to increase the performance of
vector loads and stores. The kernel uses an implicit alignmenet
of 8 bytes for the vector registers, set the alignment hint to 3.

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