If you’re working on a Mac/Win, then a cheap and cheerful way to get an rPI-compatible environment is to use Docker for Mac/Win, which transparently do CPU emulation via
qemu-user-static and binfmt.
$ docker run -it ocaml/opam2-staging:debian-9-ocaml-4.08-linux-arm32v7
opam@fdd0cc1ae238:~/opam-repository$ uname -a
Linux fdd0cc1ae238 4.9.125-linuxkit #1 SMP Fri Sep 7 08:20:28 UTC 2018 armv7l GNU/Linux
opam@fdd0cc1ae238:~/opam-repository$ ocamlc -config|grep arch
opam@fdd0cc1ae238:~/opam-repository$ opam install -y dune
You can save the binary results of the build by adding a
-v <yourrepo>:/home/opam/src to mount a local volume within the ARM container. This will be a pretty slow option, but probably faster than compiling within the rPi3.