s390/vdso: Use one large alternative instead of an alternative branch

Replace the alternative branch with a larger alternative that contains
both paths. That way the two paths are closer together and it is easier
to change both paths if the need should arise.

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