emscripten: minor refactoring

- Remove the use of EXPORTED_FUNCTIONS in favor of EMSCRIPTEN_KEEPALIVE
- SDL_Delay is no longer aliased to emscripten_sleep

src/sys/nact.cpp

@@ -489,7 +489,11 @@ void NACT::wait_after_open_menu()
 void NACT::sys_sleep(int ms) {
 	pump_events();
 	ags->update_screen();
+#ifdef __EMSCRIPTEN__
+	emscripten_sleep(ms);
+#else
 	SDL_Delay(ms);
+#endif
 }
 
 // WinMainとのインターフェース