TODO: Some of the compatibility macros are not placed in the same headers as in upstream OCaml

[kit-ty-kate]

https://github.com/ocaml-multicore/ocaml-multicore/pull/590#pullrequestreview-698356516

This breaks ocamlnet if I recall correctly

[abbysmal]

With #600 merged, I think this can be closed?

[kit-ty-kate]

caml_allocated_words was also defined in minor_gc.

I'll have a diff in a bit to see if there are more differences