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_test.go

@@ -0,0 +1,57 @@
+// 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
+
+package unix_test
+
+// Modified from Linux tests for device numbers.
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestDevices(t *testing.T) {
+	testCases := []struct {
+		path  string
+		major uint32
+		minor uint32
+	}{
+		// Device nums found using ls -l on z/OS
+		{"/dev/null", 4, 0},
+		{"/dev/zero", 4, 1},
+		{"/dev/random", 4, 2},
+		{"/dev/urandom", 4, 2},
+		{"/dev/tty", 3, 0},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s %v:%v", tc.path, tc.major, tc.minor), func(t *testing.T) {
+			var stat unix.Stat_t
+			err := unix.Stat(tc.path, &stat)
+			if err != nil {
+				if err == unix.EACCES {
+					t.Skip("no permission to stat device, skipping test")
+				}
+				t.Errorf("failed to stat device: %v", err)
+				return
+			}
+
+			dev := uint64(stat.Rdev)
+			if unix.Major(dev) != tc.major {
+				t.Errorf("for %s Major(%#x) == %d, want %d", tc.path, dev, unix.Major(dev), tc.major)
+			}
+			if unix.Minor(dev) != tc.minor {
+				t.Errorf("for %s Minor(%#x) == %d, want %d", tc.path, dev, unix.Minor(dev), tc.minor)
+			}
+			if unix.Mkdev(tc.major, tc.minor) != dev {
+				t.Errorf("for %s Mkdev(%d, %d) == %#x, want %#x", tc.path, tc.major, tc.minor, unix.Mkdev(tc.major, tc.minor), dev)
+			}
+		})
+
+	}
+}