I've updated the verbiage on the Library_Submissions process to capture a bit more about the effective role of the core libraries. It was rather messy before.
I do agree we do probably want to do more to break out what portions of the core-libraries are intended for portable implementation and which ones are just things we simply have to maintain for glue, but for that we'd probably need more feedback from other compiler implementors to get a sense of what it is they have been able / willing to implement.
-Edward