Add PCL type-forwarders.

* mcs/class/TypeForwarders/: Add new type-forwarder only assemblies.

* mcs/class/System.Core/Assembly/TypeForwarder.cs: Add type-forwarders.

* mcs/class/System.ServiceModel.Web/Assembly/TypeForwarders.cs: Likewise.

* mono/metadata/assembly.c (mono_assembly_remap_version): Remap PCL
  assemblies with different public-key tokens.