s390x: unify header guards

Standardize header guards to _ASMS390X_HEADER_H_, _S390X_HEADER_H_,
and S390X_HEADER_H, respectively.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210609143712.60933-7-cohuck@redhat.com>