According to http://crossqa.subdivi.de/src/openjdk-11 there is one missing issue to cross build OpenJDK itself (#925467, please make ant and ant-optional M-A: foreign). Note that the Debian tracker gives this M-A as well.
Looking further, we should make other build systems like gradle and groovy M-A: foreign as well, together with *all* their binary-indep dependencies. And probably mark every binary-indep package as M-A: foreign ... Time to add that to the java policy? Maybe not now, but maybe for bullseye. But a fix for ant would be appreciated now. Matthias
