A convincing concurrency story is the one offered by the two languages you mentioned: Go (with memory sharing, which is an advantage or a drawback depending on your goals), and Elixir (no memory sharing between lightweight processes).
A version of OCaml offering a similar "concurrency story" would have a lot of appeal.
Those are just techniques, other techniques achieve the same goal, such as multiprogramming, which I think is better that's why I asked.
But if you want POSIX threads and lightweight threads for the sake of it, then there are several lightweight threads libraries for ocaml (LWT being the most common one). POSIX threads you have to do from C though.
A version of OCaml offering a similar "concurrency story" would have a lot of appeal.