cpu: add support for zos/s390x

This adds feature detection that works on the s390x running zos.
There are some differences with Linux (no hwcap, facilities vector available in-memory).
These changes should not affect other platforms.

Fixes golang/go#41984.

Change-Id: Ieabbfafd07367a346a4d279dde0f00aa3fc4321b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/262477
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Trust: Martin Möhrmann <moehrmann@google.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

cpu/cpu_zos.go

@@ -0,0 +1,10 @@
+// Copyright 2020 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 cpu
+
+func archInit() {
+	doinit()
+	Initialized = true
+}