The log is here:

https://kojipkgs.fedoraproject.org//work/tasks/2862/36862862/build.log

The same architecture with 1.10:

https://kojipkgs.fedoraproject.org//packages/ocaml-dune/1.10.0/5.fc31/data/logs/armv7hl/build.log

It seems as if we need to pass -fPIC to the ocamlopt invocation (or at least that may not be the solution but I'd surely like to try that). However I can't work out how to do that with this very complex build/bootstrap system.