Wasm: Support for calling emscripten_set_main_loop

[yowl]

This PR allows code to use emscripten_set_main_loop (https://emscripten.org/docs/api_reference/emscripten.h.html#browser-execution-environment).
This is possible with the .net 5 compiler and the use of e.g.

        [DllImport("*")]
        internal static extern unsafe void emscripten_set_main_loop(delegate*<void> f,  int fps, int simulate_infinite_loop);

        [UnmanagedCallersOnly(EntryPoint = "MainLoop", CallingConvention = CallingConvention.Cdecl)]
        static void MainLoop()
        {
            Console.WriteLine("Main loop");
        }

and

        emscripten_set_main_loop(&MainLoop, 0, 0);

It adds handling for TypeFlags.FunctionPointer and adds a call to RhpReversePInvokeReturn2 when the main function exits as emscripten will subsequently make calls into the main loop function and the thread state needs to go back to preemptive.

[jkotas]

Thanks