Add Console output support for iOS

[marek-safar]

Console API is still useful even on iOS where there is no terminal available. The solution we could use is to call os_log_create with console category. We have done something similar in Mono for such purposes https://github.com/mono/mono/blob/master/mcs/class/corlib/System/Console.iOS.cs but that implementation calls to xamarin_log. It'd be preferable to have the console output working without additional dependencies.

The implementation should live inside https://github.com/dotnet/runtime/tree/master/src/libraries/System.Console/src/System

[marek-safar]

@vargaz could you look into implementing this?