Add QEMU workflows for esoteric architectures

[jmid]

To collect the 2 CI systems into one, we should consider using QEMU (or an action such as https://github.com/uraimo/run-on-arch-action using it) to test on s390x, PPC, Risc-V, and (Linux) ARM64.

Last year, I played a bit with this to stress the (yet not targeted) Risc-V backend.
My experiments are available on the riscv-attempt2 branch: https://github.com/ocaml-multicore/multicoretests/tree/riscv-attempt2

Since #473 we have a common github/runner.sh script which should help avoid some of the duplication needed to pull this off.