On main branches the GENERIC kernel contains debugging flags (e.g.
INVARIANTS, WITNESS), and we provide a GENERIC-NODEBUG kernel with those
disabled.
On stable branches the GENERIC kernel does not contain debugging flags,
and we didn't provide a -DEBUG variant. These variants are often very
useful for debugging (and generally a good idea for CI runs).
Note: this only adds amd64 for now, and is intended as a start for
discussion rather than a finished commit. This patch is against stable/14, not main