util/linuxfw, feature/buildfeatures: add ts_omit_iptables to make IPTables optional

Updates #12614

Change-Id: Ic0eba982aa8468a55c63e1b763345f032a55b4e2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/linuxfw/iptables_disabled.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (linux && !(arm64 || amd64)) || ts_omit_iptables
+
+package linuxfw
+
+import (
+	"errors"
+
+	"tailscale.com/types/logger"
+)
+
+func detectIptables() (int, error) {
+	return 0, nil
+}
+
+func newIPTablesRunner(logf logger.Logf) (*iptablesRunner, error) {
+	return nil, errors.New("iptables disabled in build")
+}