[hack] add target to build debug version of binaries
[hack] add target to build debug version of binaries

Summary:
Adding a target to build .bc executable (byte code) using dune, to be
able to use ocamldebug for example.

Reviewed By: CatherineGasnier

Differential Revision: D14638835

fbshipit-source-id: 844733e7e2bc002a12caba84c7ce65187976cead

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow