configure: always define ARCH_INT64_TYPE in config/m.h because Coq
      and maybe others rely on it
    byterun/config.h: select "long long" in preference to "long" for "int64",
      just because this is how it was done in earlier versions.
      (Minimizing suprises.)
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14635 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02