softfloat: Implement float128_(min|minnum|minnummag|max|maxnum|maxnummag)

The float128 implementation is straight-forward.
Unfortuantely, we don't have any tests we can simply adjust/unlock.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210517142739.38597-24-david@redhat.com>
[rth: Update for changed parts_minmax return value]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>