One thing I keep meaning to mention and forgetting. I’m using IcedTea’s bootstrap JDK and plug replacements for my PPC builds:

hg clone http://icedtea.classpath.org/hg/icedtea
cd icedtea
./configure
make boot plugs

The tricky bit is if you want to build 64-bit. I couldn’t decide on the best way to set the data model — integrating setarch and ARCH_DATA_MODEL and who knows what else — so I deferred that particular decision. I made it so that IcedTea always builds 32-bit on ppc and s390, which means you need to symlink the libraries if you want to build with ARCH_DATA_MODEL=64:

ln -s ppc bootstrap/jdk1.7.0/jre/lib/ppc64

I should figure this out properly but it’s hardly urgent.