Fix `ocaml-base-compiler.5.0.0~beta1` on bytecode-only architectures

[dra27]

This allows the present version of ocaml-base-compiler.5.0.0~beta1 to install. I'm tempted to say we should remove the dependency on ocaml-option-bytecode-only instead until we finally decide to merge the +options package and ocaml-base-compiler into 1.

• [x] Checked on i686 Debian ocaml-base-compiler.5.0.0~beta1 pulls in the bytecode-only package and doesn't allow its removal
• [x] Checked on amd64 Ubuntu ocaml-base-compiler.5.0.0~beta1 doesn't pull in ocaml-option-bytecode-only and it can't be added to the switch
• [x] Checked on amd64 Ubuntu ocaml-base-compiler.4.14.0 likewise doesn't pull in the package and doesn't allow its installation
• [x] Checked on i686 Debian ocaml-base-compiler.4.14.0 likewise doesn't pull in the package and doesn't allow its installation