s390: introduce register pair union

Introduce a register pair union, which is supposed to be used for
inline assemblies where instructions require parameters in even/odd
numbered register pairs.

This is more or less the same register pair construct which was
available for 31 bit builds which was removed with commit 5a79859ae0f3
("s390: remove 31 bit support").

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>