s390x/tcg: Simplify vfc64() handling

Pass the m5 field via simd_data() and don't provide specialized handlers
for single-element variants.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-6-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>