Allow finalizers on custom blocks in the minor heap, fix pr 3612

This subsume the pull request https://github.com/ocaml/ocaml/pull/82

Note that this also make functions like Bigarray.Array1.sub a lot faster:

````
  for i = 0 to n do
    ignore(Array1.sub a 3 5)
  done
````
Is more than twice faster.
Could you add that for "the finalizer of the others will be called during mark_sweep when they became unreachable?". I just forget that custom_block are not finalized as usual ocaml values....