declare some externals in .ml file in dune library "coqrun"

this seems to be enough to ensure the C code gets included in the cmxs

kernel/byterun/rocq_vm_externals.ml

@@ -0,0 +1,35 @@
+(************************************************************************)
+(*         *      The Rocq Prover / The Rocq Development Team           *)
+(*  v      *         Copyright INRIA, CNRS and contributors             *)
+(* <O___,, * (see version control and CREDITS file for authors & dates) *)
+(*   \VV/  **************************************************************)
+(*    //   *    This file is distributed under the terms of the         *)
+(*         *     GNU Lesser General Public License Version 2.1          *)
+(*         *     (see LICENSE file for the text of the license)         *)
+(************************************************************************)
+
+[@@@warning "-32"]
+
+(** We need to declare an external from each C file to convince ocaml
+    to include the C code in the cmxs for dynlink.
+
+    NB: it seems in practice we don't actually need one from each C
+    file, I guess if we have a.c and b.c and declare an external from
+    b.c whose implementation involves a.c it is enough to work.
+*)
+
+(* rocq_fix_code.c *)
+external accumulate : unit -> Obj.t = "rocq_accumulate"
+
+(* rocq_interp.c *)
+external push_val : Obj.t -> unit = "rocq_push_val"
+
+(* rocq_values.c *)
+external is_accumulate : Obj.t -> bool = "rocq_is_accumulate_code"
+
+(* rocq_float64.c *)
+external mul : float -> float -> float = "rocq_fmul_byte" "rocq_fmul"
+[@@unboxed] [@@noalloc]
+
+(* rocq_memory.c *)
+external init_vm : unit -> unit = "init_rocq_vm"