cmd/cgo: using __typeof__(a->r) instead of putting invalid TYPE in "a->r = (const TYPE)"

Thanks kballard for the hint.
Fixes #4857.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/9649045

misc/cgo/test/issue4857.go

+// 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 cgotest
+
+/*
+#cgo CFLAGS: -Werror
+const struct { int a; } *issue4857() { return (void *)0; }
+*/
+import "C"
+
+func test4857() {
+	_ = C.issue4857()
+}

src/cmd/cgo/out.go

@@ -490,7 +490,7 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	if t := n.FuncType.Result; t != nil {
 		fmt.Fprintf(fgcc, "a->r = ")
 		if c := t.C.String(); c[len(c)-1] == '*' {
-			fmt.Fprintf(fgcc, "(const %s) ", t.C)
+			fmt.Fprint(fgcc, "(__typeof__(a->r)) ")
 		}
 	}
 	fmt.Fprintf(fgcc, "%s(", n.C)