Implementation for `epoll_data` differs from Linux

Hi, the [epoll_data in Linux](https://man7.org/linux/man-pages/man3/epoll_event.3type.html) is defined as a union but wasix-libc [defines](https://github.com/wasix-org/wasix-libc/blob/f7c920f1ba8f9c9e09a77318dcb7ad01d911f7ff/libc-bottom-half/headers/public/wasi/api_wasix.h#L3537) it as a struct. Is it okay to fill the other fields or it should be approached as if its a union?