Unwind the stack for caml_stash_backtrace

[cgaebel]

In hacky ocaml-exception-detection mode, unwind the stack one more level for caml_stash_backtrace. I did some digging in core_unix and I'm not actually sure when this changed, but traces are stairstep for the following trivial async program without this quick patch.

open! Core
open! Async

let rec go () =
    Reader.open_file "a.txt"
    >>= fun a ->
    Writer.open_file "b.txt"
    >>= fun b ->
    let p1 = Reader.lines a in
    Pipe.iter p1 ~f:(fun line ->
        Writer.write_line b line;
        Writer.flushed b)
    >>= fun () ->
    Reader.close a
    >>= fun () ->
    Writer.close b
    >>= fun () ->
    Unix.rename ~src:"b.txt" ~dst:"a.txt"
    >>= fun () ->
    go ()

let command =
    Command.async ~summary:"" (Command.Param.return go)

let () =
  Command_unix.run command

If you run this, you'll need to put some lines of text in a.txt before running it.