s390x/tcg: Fix FP CONVERT TO (LOGICAL) FIXED NaN handling

In case we encounter a NaN, we have to return the smallest possible
number, corresponding to either 0 or the maximum negative number. This
seems to differ from IEEE handling as implemented in softfloat, whereby
we return the biggest possible number.

While at it, use float32_to_uint64() in the CLGEB handler.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-2-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>