stdio stdin/stdout/stderr should be in separate files
This will help static linking especially if #352 (closed) is done. Also do #350 (closed) along this work. musl does this. To get the static linking benefits, we need to remove stdin/stdout/stderr from the stdio file linked list. Instead we should just handle them specially in exit. But then exit pulls in stdin/stdout/stderr, so those should be weak aliases or something so they are only used if otherwise pulled in.