WIP dllimport for threads.cmxa as well

- st_stubs.c removes dllimport for native code in order to have a more
  efficient bridge to the runtime (is this done for primitives as
  well?). That's definitely a strong case for threads.cmxs (because
  it's different)
- ocamlnat always compiles a .cmxs for a #load of a .cmxa/.cmx:
  - For .cmxa it would seem better to load a .cmxs if it exists at the
    same location?
  - Check our META files (and down) - what's ocamlnat support like, and
    should we be listing .cmxs files here?

otherlibs/systhreads/st_stubs.c

@@ -15,7 +15,7 @@
 
 #define CAML_INTERNALS
 
-#if defined(_WIN32) && !defined(NATIVE_CODE) && !defined(_MSC_VER)
+#if defined(_WIN32) /*&& !defined(NATIVE_CODE)*/ && !defined(_MSC_VER)
 /* Ensure that pthread.h marks symbols __declspec(dllimport) so that they can be
    picked up from the runtime (which will have linked winpthreads statically).
    mingw-w64 11.0.0 introduced WINPTHREADS_USE_DLLIMPORT to do this explicitly;