gokrazy{,/natlabapp.arm64}: start adding arm64 appliance support

Both for Raspberry Pis, and for running natlab tests faster on Apple
Silicon Macs without emulating x86.

Not fully wired up yet.

Updates #1866
Updates #13038

Change-Id: I1552bf107069308f325f640773cc881ed735b5ab
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/natlabapp.arm64/config.json

@@ -0,0 +1,27 @@
+{
+    "Hostname": "natlabapp",
+    "Update": {
+        "NoPassword": true
+    },
+    "SerialConsole": "ttyS0,115200",
+    "GokrazyPackages": [
+        "github.com/gokrazy/gokrazy/cmd/dhcp"
+    ],
+    "Packages": [
+        "github.com/gokrazy/serial-busybox",
+        "tailscale.com/cmd/tailscale",
+        "tailscale.com/cmd/tailscaled",
+        "tailscale.com/cmd/tta"
+    ],
+    "PackageConfig": {
+        "tailscale.com/cmd/tailscale": {
+            "ExtraFilePaths": {
+                "/usr": "usr-dir"
+            }
+        }
+    },
+    "KernelPackage": "github.com/gokrazy/kernel.arm64",
+    "FirmwarePackage": "github.com/gokrazy/kernel.arm64",
+    "EEPROMPackage": "",
+    "InternalCompatibilityFlags": {}
+}