[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://golang.org/cl/174160043

src/runtime/softfloat_arm.go

@@ -605,10 +605,11 @@ done:
 }
 
 //go:nosplit
-func _sfloat2(pc uint32, regs *[15]uint32) {
+func _sfloat2(pc uint32, regs [15]uint32) (newpc uint32) {
 	systemstack(func() {
-		pc = sfloat2(pc, regs)
+		newpc = sfloat2(pc, &regs)
 	})
+	return
 }
 
 func _sfloatpanic()