Consider marking linked assemblies on the output

As far as I can tell linker doesn't mark the assemblies which it linked (as in changed) in any way. Some tools might want to detect linked applications to avoid problems with missing APIs. One such example is the [startup hook feature](https://github.com/dotnet/runtime/blob/master/docs/design/features/host-startup-hook.md).

One such mechanism could be to inject an assembly level attribute, something like:
```C#
[assembly: AssemblyMetadata("ILLinkProcessed", "True")]
```

Size impact of such change should be trivial.