cpu: get hwcap/auxv from the Go 1.21+ runtime

Depends on https://go.dev/cl/458256

This change only does Linux for now.

Updates golang/go#57336

Change-Id: I0659697c1bdc6e2577c6251b964a0df32047ee12
Reviewed-on: https://go-review.googlesource.com/c/sys/+/465295
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

cpu/runtime_auxv_go121_test.go

@@ -0,0 +1,21 @@
+// Copyright 2023 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
+// +build go1.21
+
+package cpu
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestAuxvFromRuntime(t *testing.T) {
+	got := getAuxv()
+	t.Logf("got: %v", got) // notably: we didn't panic
+	if runtime.GOOS == "linux" && len(got) == 0 {
+		t.Errorf("expected auxv on linux; got zero entries")
+	}
+}