internal/syscall/unix: added support for linux/mips64{,le}

Change-Id: Ib6f168b6a2daf3befbf75b4b95967ff91ac95d12
Reviewed-on: https://go-review.googlesource.com/14456Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/syscall/unix/getrandom_linux_mips64x.go

+// 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.
+
+// +build mips64 mips64le
+
+package unix
+
+const randomTrap uintptr = 5313