Temporarily disable sorting of function symbols by debug info. Emit function symbols in topological order.

Currently, the layout of function symbols is determined by their debug info, so that functions in the final assembly file appear in the same order as in the source file. This is convenient for debugging and stable layout. However, it can makes some backend passes overly conservative. Currently, this affects only checkmach and polling that rely on "future" function names. For example, zero_alloc check fails on a common code pattern raise_s [%message ...].

let[@zero_alloc] raise_invalid_index ~index ~name =
  if index < 0 then
  raise_s
    [%message
      "Negative" (index : int) (name : string)]

results in the following code after ppx :

let raise_invalid_index ~index ~name =
  if index < 0 then
  raise_s
    (let ppx_sexp_message () =
       Sexp.List
         [Sexp.Atom "Negative";
          Sexp.List
            [Sexp.Atom "index"; Sexp.of_int index];
          Sexp.List
            [Sexp.Atom "name"; Sexp.Atom name]]
     [@@ocaml.inline never][@@ocaml.local never][@@ocaml.specialise never]
     in
     ((ppx_sexp_message ())[@nontail ]))
[@@ocaml.inline never][@@ocaml.local never][@@ocaml.specialise never]

and raise_invalid_index is emitted before ppx_sexp_message even though they are not mutually recursive.

A proper fix is more involved. The backend translates functions in layout order, fully translating each function from Cmm all the way to assembly before moving on to the next function. We can change the backend to translate the functions in topological order and then emit them in source order. This means holding on to Mach.fundecl longer and careful handling any mutable state. Separately, an improvenent to zero_alloc analysis to handle mutually recursive functions will also fix this.