rt: don't save and restore xmm/regs in __morestack.
auto merge of #5418 : luqmana/rust/stack-float, r=brson

Like I commented in #2043, I can't reproduce the weirdness from #1388 on either mac or linux (x84_64) and pushing to try gives all green.

That's 128 less bytes to have to keep in the stack for every call to __morestack.