partially fix the dune build

asmcomp/debug was removed by ocaml#10355

There is still a failure on my machine:

    File "asmcomp/emitaux.mli", line 88, characters 31-55:
    Error: Unbound module Emitenv

asmcomp/mitenv is a .mli-only module that was apprently introduced in ocaml#8936.

dune

@@ -26,7 +26,6 @@
 (copy_files# bytecomp/*.ml{,i})
 (copy_files# driver/*.ml{,i})
 (copy_files# asmcomp/*.ml{,i})
-(copy_files# asmcomp/debug/*.ml{,i})
 (copy_files# file_formats/*.ml{,i})
 (copy_files# lambda/*.ml{,i})
 (copy_files# middle_end/*.ml{,i})