Compiler flags instead of attributes in stdlib.ml can be used with Dune 3.7.1

[gretay-js]

Dune 3.7.1 includes the fix from ocaml/dune#7241. When we move to this dune, the attribute [@@@zero_alloc check] introduced by #1294 in ocaml/stdlib can be removed, and maybe [@@@ ocaml.flambda_o3] can be replace with a compiler flag. Using the attributes makes the diff with upstream bigger although not hard to maintian.