[mscorlib] Add support for intercepting threadpool work, and use it for Xamarin.iOS...

[mscorlib] Add support for intercepting threadpool work, and use it for Xamarin.iOS/Mac.

This is used to Xamarin.iOS/Mac to wrap threadpool work in an NSAutoreleasePool.

Also open up corlib's internals to Xamarin.iOS/Mac so that the linker
doesn't have to be involved when accessing internal API.