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/parse.go

@@ -0,0 +1,43 @@
+// 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.
+
+package cpu
+
+import "strconv"
+
+// parseRelease parses a dot-separated version number. It follows the semver
+// syntax, but allows the minor and patch versions to be elided.
+//
+// This is a copy of the Go runtime's parseRelease from
+// https://golang.org/cl/209597.
+func parseRelease(rel string) (major, minor, patch int, ok bool) {
+	// Strip anything after a dash or plus.
+	for i := 0; i < len(rel); i++ {
+		if rel[i] == '-' || rel[i] == '+' {
+			rel = rel[:i]
+			break
+		}
+	}
+
+	next := func() (int, bool) {
+		for i := 0; i < len(rel); i++ {
+			if rel[i] == '.' {
+				ver, err := strconv.Atoi(rel[:i])
+				rel = rel[i+1:]
+				return ver, err == nil
+			}
+		}
+		ver, err := strconv.Atoi(rel)
+		rel = ""
+		return ver, err == nil
+	}
+	if major, ok = next(); !ok || rel == "" {
+		return
+	}
+	if minor, ok = next(); !ok || rel == "" {
+		return
+	}
+	patch, ok = next()
+	return
+}