softfloat: Fix division

softfloat: Fix division

The __udiv_qrnnd primitive that we nicked from gmp requires its
inputs to be normalized.  We were not doing that.  Because the
inputs are nearly normalized already, finishing that is trivial.

Replace div128to64 with a "proper" udiv_qrnnd, so that this
remains a reusable primitive.

Fixes: cf07323d494
Fixes: https://bugs.launchpad.net/qemu/+bug/1793119
Tested-by: Emilio G. Cota <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>