Use executable copy of the FFI closure
Use executable copy of the FFI closure

Modern CPUs restrict executing modifiable memory. Lib_ffi deals with this by mapping to copies of the closure object into memory. This change ensures that we use the executable copy for the function pointer.
yallop/ocaml-ctypes,yallop/ocaml-ctypes,ocamllabs/ocaml-ctypes,ocamllabs/ocaml-ctypes