utils/gcm-ref (poly64_mul_simple): Strip padding off the product.

utils/gcm-ref (poly64_mul_simple): Strip padding off the product.

Rather than leaving this job to the caller.  I'm going to decree that
it's the presentation-function's job to show padding in the right place,
rather than the multiplier's job to retain it.  This means that we need
to keep track of the padding properly, but it's pretty easy.

The most important effect is that there's no longer a rather strange
bodge in `poly64_common' to strip the padding in one particular case
because `poly64_mul_simple' has done it properly in every case.