tailcfg,hostinfo: add Hostinfo.Machine and Hostinfo.GoArchVar

For detecting a non-ideal binary running on the current CPU.

And for helping detect the best Synology package to update to.

Updates #6995

Change-Id: I722f806675b60ce95364471b11c388150c0d4aea
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo_uname.go

@@ -0,0 +1,39 @@
+// Copyright (c) 2023 Tailscale Inc & 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 || freebsd || openbsd || darwin
+
+package hostinfo
+
+import (
+	"runtime"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/types/ptr"
+)
+
+func init() {
+	unameMachine = lazyUnameMachine.Get
+}
+
+var lazyUnameMachine = &lazyAtomicValue[string]{f: ptr.To(unameMachineUnix)}
+
+func unameMachineUnix() string {
+	switch runtime.GOOS {
+	case "android":
+		// Don't call on Android for now. We're late in the 1.36 release cycle
+		// and don't want to test syscall filters on various Android versions to
+		// see what's permitted. Notably, the hostinfo_linux.go file has build
+		// tag !android, so maybe Uname is verboten.
+		return ""
+	case "ios":
+		// For similar reasons, don't call on iOS. There aren't many iOS devices
+		// and we know their CPU properties so calling this is only risk and no
+		// reward.
+		return ""
+	}
+	var un unix.Utsname
+	unix.Uname(&un)
+	return unix.ByteSliceToString(un.Machine[:])
+}