wgengine/netstack: fix 32-bit build broken from prior commit

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

Makefile

@@ -15,7 +15,10 @@ depaware:
 buildwindows:
 	GOOS=windows GOARCH=amd64 go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
 
-check: staticcheck vet depaware buildwindows
+build386:
+	GOOS=linux GOARCH=386 go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
+
+check: staticcheck vet depaware buildwindows build386
 
 staticcheck:
 	go run honnef.co/go/tools/cmd/staticcheck -- $$(go list ./... | grep -v tempfork)

wgengine/netstack/netstack_32bit.go

@@ -8,14 +8,30 @@
 package netstack
 
 import (
+	"context"
 	"errors"
+	"net"
 
+	"inet.af/netaddr"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/magicsock"
 	"tailscale.com/wgengine/tstun"
 )
 
-func Create(logf logger.Logf, tundev *tstun.TUN, e wgengine.Engine, mc *magicsock.Conn) (wgengine.FakeImpl, error) {
+type Impl struct{}
+
+func (*Impl) Start() error { panic("noimpl") }
+
+func (*Impl) DialContextTCP(ctx context.Context, addr string) (net.Conn, error) { panic("noimpl") }
+
+type DNSMap map[string]netaddr.IP
+
+func (m DNSMap) Resolve(ctx context.Context, addr string) (netaddr.IPPort, error) { panic("noimpl") }
+
+func DNSMapFromNetworkMap(nm *netmap.NetworkMap) DNSMap { return nil }
+
+func Create(logf logger.Logf, tundev *tstun.TUN, e wgengine.Engine, mc *magicsock.Conn) (*Impl, error) {
 	return nil, errors.New("netstack is not supported on 32-bit platforms for now; see https://github.com/google/gvisor/issues/5241")
 }