std.Io.Threaded: handle systems that don't have preadv

Extracted from https://github.com/ziglang/zig/pull/25592.

```zig
const have_preadv = switch (native_os) {
    .windows, .haiku, .serenity => false, // 💩💩💩
    else => true,
};
```

```zig
    if (!have_preadv) @compileError("TODO");
```

Windows has its own implementation of `fileReadPositional` so this is a task for haiku and serenity only.