s390x: Fix library constant definitions

Seems like I uppercased the whole region instead of only the ULs when
I added those definitions. Let's make the x lowercase again.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200429143518.1360468-11-frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>