runtime: convert int64tofloat64, uint64tofloat64 to Go

I noticed that 5g doesn't flush the float64 result back to the stack, hence the change in the function signature. I'm wondering if I should also change the signature for the other two functions.

LGTM=rsc
R=minux, josharian, rsc
CC=golang-codereviews
https://codereview.appspot.com/132990044