[tests] Fix some corlib unit tests to run successfully on iOS
[PCL]: Add "System.Dynamic.Runtime.dll" Facade Assembly to MonoTouch.

Added the type-forwarder for `ConditionalWeakTable<T,V>', put all the
others inside a `!FULL_AOT_RUNTIME' conditional, then enabled this in
the MonoTouch build.
[corlib]: Add 'System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken' stub.
[PCL]: Add new 'System.Runtime.InteropServices.WindowsRuntime' Facade Assembly.

FIXME: This is only a stub, the only type currently present is
       `System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken',
       which is only a stub.
[corlib]: Add stubs for the remaining `System.Runtime.InteropServices.WindowsRuntime' APIs.