wgengine/netstack: don't build netstack on 32-bit platforms

See google/gvisor#5241

wgengine/netstack/netstack.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// netstack doesn't build on 32-bit machines (https://github.com/google/gvisor/issues/5241)
+// +build amd64 arm64 ppc64le riscv64 s390x
+
 // Package netstack wires up gVisor's netstack into Tailscale.
 package netstack
 

wgengine/netstack/netstack_32bit.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2020 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.
+
+// netstack doesn't build on 32-bit machines (https://github.com/google/gvisor/issues/5241)
+// +build !amd64,!arm64,!ppc64le,!riscv64,!s390x
+
+package netstack
+
+import (
+	"errors"
+
+	"tailscale.com/types/logger"
+	"tailscale.com/wgengine"
+	"tailscale.com/wgengine/magicsock"
+	"tailscale.com/wgengine/tstun"
+)
+
+func Impl(logf logger.Logf, tundev *tstun.TUN, e wgengine.Engine, mc *magicsock.Conn) error {
+	return errors.New("netstack is not supported on 32-bit platforms for now; see https://github.com/google/gvisor/issues/5241")
+}