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

[andrewrk]

Extracted from #25592.

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

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

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

[The-King-of-Toasters]

Haiku has preadv at home.