Add persistent USB output mode selection

tests/mode_native_stubs/hardware/structs/watchdog.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stdint.h>
+
+struct watchdog_hw_t {
+    uint32_t scratch[8];
+};
+
+extern watchdog_hw_t* watchdog_hw;

tests/mode_native_stubs/hardware/watchdog.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include <stdint.h>
+
+void watchdog_reboot(uint32_t pc, uint32_t sp, uint32_t delay_ms);