cmd/gc: fix comparison order of parameters in mpcmpfltc(a, b)

It should compare a - b to 0, not b - a to 0.
Fixes #6964.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/39020044

src/cmd/gc/mparith1.c

@@ -65,7 +65,7 @@ mpcmpfltc(Mpflt *b, double c)
 	Mpflt a;
 
 	mpmovecflt(&a, c);
-	return mpcmpfltflt(&a, b);
+	return mpcmpfltflt(b, &a);
 }
 
 void

test/fixedbugs/issue6964.go

@@ -0,0 +1,11 @@
+// errorcheck
+
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+	_ = string(-4 + 2i + 2) // ERROR "-4\+2i"
+}