[nostalgia] Make core::shutdown return void and add GBA implementation

src/nostalgia/core/gba/core.arm.cpp

@@ -20,7 +20,7 @@ UpdateHandler g_updateHandler = nullptr;
 
 ox::Error run(Context *ctx) noexcept {
 	g_wakeupTime = 0;
-	while (1) {
+	while (ctx->running) {
 		if (g_wakeupTime <= g_timerMs && g_updateHandler) {
 			auto sleepTime = g_updateHandler(ctx);
 			if (sleepTime >= 0) {

src/nostalgia/core/gba/core.cpp

@@ -59,4 +59,8 @@ bool buttonDown(Context*, Key k) noexcept {
 	return !(REG_GAMEPAD & (1 << static_cast<int>(k)));
 }
 
+void shutdown(Context *ctx) noexcept {
+	ctx->running = false;
+}
+
 }