Updating loading mechanism of native libdl in OpenGL.NET

#if NETCORE

    [DllImport("dl")]
    public static extern IntPtr dlopen(string filename, int flags);

    [DllImport("dl")]
    public static extern IntPtr dlsym(IntPtr handle, string symbol);

#else

    [DllImport("dl")]
    public static extern IntPtr dlopen([MarshalAs(UnmanagedType.LPTStr)] string filename, int flags);

    [DllImport("dl")]
    public static extern IntPtr dlsym(IntPtr handle, [MarshalAs(UnmanagedType.LPTStr)] string symbol);

#endif

    [DllImport("dl")]
    public static extern string dlerror();    }