Building the firebuild package for riscv64 and other ports

[Bálint Réczey <balint@balintreczey.hu>]

Hi,

Following the relevant Policy section [1] I have marked firebuild [2]
to be auto buildable and buildd admins kindly added the package to the
allowlist. However, the allowlist does not seem to work for ports,
just release architectures.
Would it be possible to put firebuild on the allowlist for ports, too?
It already builds in Ubuntu for riscv64 and the package being a build
accelerator could be quite handly on slower architectures.

Cheers,
Balint

[1] https://www.debian.org/doc/manuals/developers-reference/pkgs.html#non-free-buildd
[2] https://tracker.debian.org/pkg/firebuild