s390x/tcg: Implement 128 bit for VECTOR FP LOAD ROUNDED

128 bit -> 64 bit, there is only a single element to process.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-19-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>