[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

facebook/flow,facebook/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,gabelevi/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,gabelevi/flow,mroch/flow,gabelevi/flow,mroch/flow,mroch/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,facebook/flow,mroch/flow