unix: add support for zos/s390x

This adds sys/unix support for zos/s390x.
These changes should not affect other platforms.

Fixes golang/go#43230

Change-Id: If1c1d9df134fbc44f95ca1b214e836508696da93
Reviewed-on: https://go-review.googlesource.com/c/sys/+/275992
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/dev_zos.go

@@ -0,0 +1,29 @@
+// 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.
+
+//go:build zos && s390x
+// +build zos,s390x
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used by z/OS.
+//
+// The information below is extracted and adapted from <sys/stat.h> macros.
+
+package unix
+
+// Major returns the major component of a z/OS device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev >> 16) & 0x0000FFFF)
+}
+
+// Minor returns the minor component of a z/OS device number.
+func Minor(dev uint64) uint32 {
+	return uint32(dev & 0x0000FFFF)
+}
+
+// Mkdev returns a z/OS device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	return (uint64(major) << 16) | uint64(minor)
+}