How to use TypeForwardedTo in Portable Class Libraries?

How to use TypeForwardedTo in Portable Class Libraries?

I'm trying to build a Portable Class Library which uses implementations
from the platform when available. For example, Lazy<T> is available on
.NET 4.5, Windows Store Apps, Windows Phone 8, BUT it's not available on
Windows Phone 7, Silverlight 4. When my PCL is loaded on one of the
platforms that has a Lazy<T> implementation, I want to use the platform's
implementation. When it's not available on the platform, I want to use my
own implementation. It seems to be possible because the Microsoft BCL is
doing it, but I haven't figured out how to implement it.
I've read that by using the TypeForwardedToAttribute, you can redirect the
PCL to use the implementation from the platform. I'm not quite sure how to
configure my Visual Studio Projects to achieve this result. If CoreLib is
my library, and ShimLib contains my implementation of Lazy<T>. Where do I
add the TypeForwardedToAttribute? The attribute requires an actual Type
reference typeof(System.Lazy<>), which doesn't work when Windows Phone 7
is targeted in the PCL. If I remove Windows Phone 7, then I can't add a
reference from CoreLib to ShimLib because ShimLib doesn't support all the
platforms that CoreLib does. How do I handle this?
Yes, I know Lazy<T> is super easy to implement, but it's just an example,
and my actual situation applies to many more classes that are less trivial
to implement.