unix: add Auxv

Fixes golang/go#67839

Change-Id: I3af38d21159f7cac3786b49ac17657d314fbc178
Reviewed-on: https://go-review.googlesource.com/c/sys/+/644295
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/auxv_linux_test.go

@@ -0,0 +1,23 @@
+// Copyright 2025 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 go1.21 && linux
+
+package unix_test
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestAuxv(t *testing.T) {
+	vec, err := unix.Auxv()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(vec) == 0 {
+		t.Errorf("got zero auxv entries on linux, expected > 0")
+	}
+}