math/mpx-mul4-*-sse2.S (squash): We don't care about the top half of c3 here.

math/mpx-mul4-*-sse2.S (squash): We don't care about the top half of c3 here.

The previous version of the comment erroneously claimed that the top
half of c3 held y_1; in fact it holds y_2, but we'll clobber it anyway
because the objective is to carry up into y_1, so mark it as
don't-care (like lo).