internal/syscall/unix: add randomTrap const for GOARCH=mips{,le}

Change-Id: I76c62a7b79ea526f59f281e933e4fd431539d2da
Reviewed-on: https://go-review.googlesource.com/31486Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/internal/syscall/unix/getrandom_linux_mipsx.go

+// 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 mips mipsle
+
+package unix
+
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
+const randomTrap uintptr = 4353