unix: validate ParseDirent inputs

This is a copy of https://golang.org/cl/23780 for the x/sys repo.

Don't panic, crash, or return references to uninitialized memory when 
ParseDirent is passed invalid input.

Updates golang/go#15653
Fixes golang/go#19754

Change-Id: Idb7cffe14d48ed662e5a55ecb5249c1907cf4003
Reviewed-on: https://go-review.googlesource.com/38758
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/endian_big.go

@@ -0,0 +1,9 @@
+// Copyright 2016 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.
+//
+// +build ppc64 s390x mips64
+
+package unix
+
+const isBigEndian = true