Returns the current platforms default library suffix:

• dll on windows
• dylib on mac
• so on linux

Useful for dlopen() code to load the correct library:

  @impl true
  def on_load() do
    """
    // loading the library with dlopen()
    gmp = dlopen("libgmp.#{library_suffix()}", RTLD_LAZY);
    if (!gmp) {
      return "could not load libgmp";
    }
    """
  end