Finish renaming rust-builtin back to rust-intrinsic

[marijnh]

I implemented the new intrinsic system under the name 'builtin' so as not to clash with the existing code. I'm about to push some changes that remove the old system and rename everything inside the compiler, but leave it recognizing both rust-builtin and rust-intrinsic as native mod api specifications. After the next snapshot, I can change the references to rust-builtin in the core lib and remove support for the rust-builtin attribute.