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

src/nostalgia/core/glfw/core.cpp

@@ -79,10 +79,9 @@ bool buttonDown(Context *ctx, Key key) noexcept {
 	return (id->keysDown >> static_cast<int>(key)) & 1;
 }
 
-ox::Error shutdown(Context *ctx) noexcept {
+void shutdown(Context *ctx) noexcept {
 	const auto id = ctx->windowerData<GlfwImplData>();
 	glfwSetWindowShouldClose(id->window, true);
-	return OxError(0);
 }
 
 }