math/mpx-mul4-x86-sse2.S: Make stack alignment more standard.

This actually slightly reduces the amount of stack needed, but I don't
quite understand why.  There's a knock-on rearrangement of the stack
frame in the test wrappers and C-interface subroutines.

There's also a slightly sneaky introduction of space for a later change.
But there shouldn't be any externally observable difference.