cctools: disconnect the llvm versions supported by cctools from the macports-clang versions the assembler will search for

[kencu (Ken)]

there is no reason these two lists should be connected, it is just a source of errors.

The macports-clang-N versions that the assembler looks for should be obtained in some other way, and can be free to include versions that don't exist yet, in anticipation of the future where they might/will exist.