Better handling of .map files in js_of_ocaml, wasm_of_ocaml

[JasonGross]

Currently, we are pulling various sed hacks to deal with .map files when bedrock2 is enabled. We should have a more systematic way of generating files with appropriate names and mappings

https://github.com/mit-plv/fiat-crypto/blob/b739724dcf585c4d85356a7772c7917db4b575f9/Makefile.standalone#L134-L212