As long as it's also capable of reproducible builds (and I would think the proofs would be very hard if the output is not even predictable!), you should be able

As long as it's also capable of reproducible builds (and I would think the proofs would be very hard if the output is not even predictable!), you should be able to cross-build it from a certified platform, and bootstrap it from a trusted one (or more than one), and the bootstraps should converge to an identical binary, thus giving you both claims.