x86: unify header guards

Standardize header guards to _ASMX86_HEADER_H_, _X86_HEADER_H_,
and X86_HEADER_H.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210609143712.60933-8-cohuck@redhat.com>