math/mpx-mul4-x86-sse2.S: Maintain a local copy of the counter.

math/mpx-mul4-x86-sse2.S: Maintain a local copy of the counter.

I've no idea whether one's allowed to mutate a parameter passed on the
stack.  Play it safe.

This means that (a) the counter is now in a fixed place in the frame so
that `testtail' doesn't need to be told where it is, an (b)
`testprologue' needs to initialize it from the caller's parameter, so it
needs to grow a macro argument.