s390x/tcg: Simplify vop64_3() handling

Let's simplify, reworking our handler generation, passing the whole "m5"
register content and not providing specialized handlers for "se", and
reading/writing proper float64 values using new helpers.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-4-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>