Compatibility for /lib64 suffix

[redin0s]

I was trying to build bap from source and I encountered an error when building.
Specifically in tools/rewrite.ml there is an issue when the ocaml library is present in the path /usr/lib64/ocaml and the following instruction raises an error.

String.chop_suffix_exn stdlib "/lib/ocaml"

I have attempted a naive fix by modifying the code.

let prefix =
  let stdlib = Cfg.ocaml_config_var_exn self "standard_library" in
  (
  try
      String.chop_suffix_exn stdlib "/lib/ocaml"
  with
  | exn ->
      String.chop_suffix_exn stdlib "/lib64/ocaml"
  ) in

I have not submitted a pull request since I don't really know if this is the standard way of dealing with this things in ocaml as it is my first time using.