cpu: parse /proc/cpuinfo on linux/arm64 on old kernels when needed

Updates tailscale/tailscale#5793
Fixes golang/go#57336

Change-Id: I4f8128bebcc58f265d447ecaaad2473aafa9131c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/458315
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cpu/proc_cpuinfo_linux.go

@@ -0,0 +1,54 @@
+// Copyright 2022 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 linux && arm64
+// +build linux,arm64
+
+package cpu
+
+import (
+	"errors"
+	"io"
+	"os"
+	"strings"
+)
+
+func readLinuxProcCPUInfo() error {
+	f, err := os.Open("/proc/cpuinfo")
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	var buf [1 << 10]byte // enough for first CPU
+	n, err := io.ReadFull(f, buf[:])
+	if err != nil && err != io.ErrUnexpectedEOF {
+		return err
+	}
+	in := string(buf[:n])
+	const features = "\nFeatures	: "
+	i := strings.Index(in, features)
+	if i == -1 {
+		return errors.New("no CPU features found")
+	}
+	in = in[i+len(features):]
+	if i := strings.Index(in, "\n"); i != -1 {
+		in = in[:i]
+	}
+	m := map[string]*bool{}
+
+	initOptions() // need it early here; it's harmless to call twice
+	for _, o := range options {
+		m[o.Name] = o.Feature
+	}
+	// The EVTSTRM field has alias "evstrm" in Go, but Linux calls it "evtstrm".
+	m["evtstrm"] = &ARM64.HasEVTSTRM
+
+	for _, f := range strings.Fields(in) {
+		if p, ok := m[f]; ok {
+			*p = true
+		}
+	}
+	return nil
+}