cmd/cgo: generate definitions for GoSlice

        Fixes #3741.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6308076

misc/cgo/test/issue3741.go

+// Copyright 2012 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
+
+import "C"
+
+//export exportSliceIn
+func exportSliceIn(s []byte) bool {
+	return len(s) == cap(s)
+}
+
+//export exportSliceOut
+func exportSliceOut() []byte {
+	return []byte{1}
+}
+
+//export exportSliceInOut
+func exportSliceInOut(s []byte) []byte {
+	return s
+}

src/cmd/cgo/out.go

@@ -958,4 +958,5 @@ typedef struct { char *p; int n; } GoString;
 typedef void *GoMap;
 typedef void *GoChan;
 typedef struct { void *t; void *v; } GoInterface;
+typedef struct { void *data; int len; int cap; } GoSlice;
 `