target/s390x: Remove leading underscores from #defines

We should not use leading underscores followed by a capital letter
in #defines since such identifiers are reserved by the C standard.

For ASCE_ORIGIN, REGION_ENTRY_ORIGIN and SEGMENT_ENTRY_ORIGIN I also
added parentheses around the value to silence an error message from
checkpatch.pl.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1520227018-4061-1-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>