Comment 1 for bug 1872931

Combined with the recent Debian changes into preliminary changeset, but still discussing if that needs an Feature Freeze exception (or not need to be in 20.04 at all and could be 20.10).

   * enable riscv build (LP: #1872931)
     - enable support for riscv64 hosts
     - only enable librbd on architectures where it is built
     - ceph: do not list librados-dev as we only use librbd-dev and the latter depends on the former
     - seccomp grew up, no need in versioned build-dep
     - enable seccomp only on architectures where it can be built