f2j fails to cross build from source, because it fails running the java
compiler which is requested for the host architecture by Build-Depends.
Before figuring out how to fix this, I'd like to observe that the
compiled jar file is installed into an arch:all package. This is not
produced by a cross build. So instead of fixing it, I propose moving
Java out of the arch-only build. This also helps build daemons on slower
architectures as installing Java can take quite some time. Please
consider applying the attached patch.

Helmut