Rename SSE -> XMM for consistency.

We were using SSE is some places and XMM in others. Better to keep a consistent
naming scheme.
Only use real XMM registers when assigning arguments.

My original change to the calling convention mistakenly used all 6 XMM
registers---which live in the global register table---on x86 (32 bit). This
royally screwed up the floating point code generated for that platform because
floating point arguments were passed in global registers instead of on the
stack!