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

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;
+}
+
 }