[xbuild]: Implicitly expand PCL2 facade assemblies.

* GetReferenceAssemblyPaths: return the reference assemblies for the
  requested PCL profile.

* ResolveAssemblyReference: always include System.Runtime as a
  dependency; the .targets check for this to determine whether
  we're a PCL2 library.
Look for facade assemblies in $(mono_libdir)/4.5/Facades/