cmd/tailscale/cli: fix ssh CLI command breaking the Wasm build

Adds a stub for syscall.Exec when GOOS=js. We also had a separate branch
for Windows, might as well use the same mechanism there too.

For #3157

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tailscale/cli/ssh_exec_windows.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2022 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.
+
+package cli
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+)
+
+func execSSH(ssh string, argv []string) error {
+	// Don't use syscall.Exec on Windows, it's not fully implemented.
+	cmd := exec.Command(ssh, argv[1:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	var ee *exec.ExitError
+	err := cmd.Run()
+	if errors.As(err, &ee) {
+		os.Exit(ee.ExitCode())
+	}
+	return err
+}