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>

cmd/tailscaled/sigpipe.go

@@ -0,0 +1,12 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build go1.21 && !plan9
+
+package main
+
+import "syscall"
+
+func init() {
+	sigPipe = syscall.SIGPIPE
+}