unix: fix infinite recursion in itoa

Bring in http://golang.org/cl/138650044 from syscall package.

Change-Id: I554b0b31b981c682b6826644564971321a55c9e4
Reviewed-on: https://go-review.googlesource.com/10030
Reviewed-by: Rob Pike <r@golang.org>

unix/export_test.go

@@ -0,0 +1,7 @@
+// Copyright 2015 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 unix
+
+var Itoa = itoa