Parallel Dynlink usage under Cygwin+MinGW is unsafe

[jmid]

We've observed an issue with Dynlink under the Cygwin and MinGW ports, which may cause segfaults, hangs, etc.
This has been tracked in ocaml-multicore/multicoretests#307. Here's a small reproducer:

libB.ml

let value = 34

main.ml

let loadfile f =
  try Dynlink.loadfile (Dynlink.adapt_filename f)
  with Dynlink.Error (Dynlink.Module_already_loaded _) -> ()

let dont_crash () =
  let wait = Atomic.make true in
  let dom1 = Domain.spawn (fun () ->
                             while Atomic.get wait do Domain.cpu_relax() done;
                             loadfile "libB.cmxs") in
  let dom2 = Domain.spawn (fun () ->
                             Atomic.set wait false;
                             loadfile "libB.cmxs") in
  let _ = Domain.join dom1 in
  let _ = Domain.join dom2 in
  ()

let _ =
  for i=1 to 1000 do
    Printf.printf "%i %!" i;
    dont_crash ()
  done

steps to reproduce:

ocamlopt -g -shared libB.ml -o libB.cmxs
ocamlopt -g -I +dynlink dynlink.cmxa repro.ml -o repro.exe
./repro.exe

Across 10 MinGW runs I observed the following behaviour (sorted). I've observed simiar under Cygwin.

Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Failure("input_value_from_block: bad object"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Failure("input_value_from_block: bad object"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Failure("input_value_from_block: bad object"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Failure("not an OCaml plugin"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Failure("not an OCaml plugin"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Invalid_argument("Dynlink: Missing frametable for LibB"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Invalid_argument("Dynlink: Missing frametable for LibB"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Invalid_argument("Dynlink: Missing frametable for _shared_startup"))
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll Invalid_argument("Dynlink: Missing gc_roots for _shared_startup"))
Segmentation fault

This is an issue on trunk, 5.2.0~alpha1, 5.1.1, ...
I've since learned that this is a known FlexDLL issue: ocaml/flexdll#120
and that #11607 disabled a compiler test under Windows as a result.
I'm therefore opening this issue to help keep track.

(As usual) @dra27 has a draft fix: a first version adds a global lock around FlexDLL'ss list of loaded units.
I can confirm that it fixes the above reproducer.

Note: an earlier PR ocaml/flexdll#112 fixed races to FlexDLL's global error variables (which is orthogonal to the above AFAICS)