all: adjust some build tags for plan9

I'm not saying it works, but it compiles.

Updates #5794

Change-Id: I2f3c99732e67fe57a05edb25b758d083417f083e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/endpoint_stub.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build wasm || plan9
+
+package magicsock
+
+// isBadEndpointErr checks if err is one which is known to report that an
+// endpoint can no longer be sent to. It is not exhaustive, but covers known
+// cases.
+func isBadEndpointErr(err error) bool {
+	return false
+}