[llvm] [CMake] Add tablegen job pool support (PR #84762)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd approved this pull request.

This seems like a good idea to me. I wonder if we should generalise this to say `CODEGEN` but, I don't think that is worth nit-picking on currently.

https://github.com/llvm/llvm-project/pull/84762