[dev.cc] runtime: fix _sfloat thunk

* _sfloat dispatches to runtime._sfloat2 with the Go calling convention, so the seecond argument is a [15]uint32, not a *[15]uint32.
* adjust _sfloat2 to return the new pc in 68(R13) as expected.

LGTM=rsc
R=minux, austin, rsc
CC=golang-codereviews
https://codereview.appspot.com/174160043