internal/syscall/unix: add randomTrap const for s390x

Change-Id: I81376f524e76db25fd52cc5bec2c80fbf618a0c5
Reviewed-on: https://go-review.googlesource.com/20877Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/syscall/unix/getrandom_linux_s390x.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.
+
+package unix
+
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
+const randomTrap uintptr = 349