gokrazy: add arm64 natlab appliance image support

Add natlabapp.arm64 config and gokrazydeps.go for building a gokrazy
natlab appliance image targeting arm64 (Apple Silicon). This is the
arm64 counterpart to the existing natlabapp (amd64) used by vmtest.

The arm64 image uses github.com/gokrazy/kernel.arm64 and is built
with "make natlab-arm64" in the gokrazy directory.

Updates #13038

Change-Id: I0e1f8e5840083a5de5954f2cf46e3babec129d96
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/Makefile

@@ -11,3 +11,8 @@ qemu: image
 natlab:
 	go run build.go --build --app=natlabapp
 	qemu-img convert -O qcow2 natlabapp.img natlabapp.qcow2
+
+# For natlab integration tests on macOS arm64:
+natlab-arm64:
+	go run build.go --build --app=natlabapp.arm64
+	qemu-img convert -O qcow2 natlabapp.arm64.img natlabapp.arm64.qcow2

gokrazy/natlabapp.arm64/gokrazydeps.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build for_go_mod_tidy_only
+
+package gokrazydeps
+
+import (
+	_ "github.com/gokrazy/gokrazy"
+	_ "github.com/gokrazy/gokrazy/cmd/dhcp"
+	_ "github.com/gokrazy/kernel.arm64"
+	_ "github.com/gokrazy/serial-busybox"
+	_ "tailscale.com/cmd/tailscale"
+	_ "tailscale.com/cmd/tailscaled"
+	_ "tailscale.com/cmd/tta"
+)