s390/vx: add 64 and 128 bit members to __vector128 struct

Add 64 and 128 bit members to __vector128 struct in order to allow reading
of the complete value, or the higher or lower part of vector register
contents instead of having to use casts.

Add an explicit __aligned(4) statement to avoid that the alignment of the
structure changes from 4 to 8. This should make sure that no breakage
happens because of this change.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>