cmd/tailscaled, net/tstun: build for aix/ppc64

At least in userspace-networking mode.

Fixes #11361

Change-Id: I78d33f0f7e05fe9e9ee95b97c99b593f8fe498f2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tstun_stub.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build plan9 || aix
+
+package tstun
+
+import (
+	"github.com/tailscale/wireguard-go/tun"
+	"tailscale.com/types/logger"
+)
+
+func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
+	panic("not implemented")
+}
+
+func Diagnose(logf logger.Logf, tunName string, err error) {
+	panic("not implemented")
+}