Can you post the build log from a full clean build? This one looks like it was restarted, and from the error messages I'm guessing the toolchain didn't compile correctly.
The fastest way to make this work for sure is to install a Debian Stretch chroot, bind mount /proc, /sys, and /dev, then build inside that chroot. I don't think IBM fully supports the newer software versions in Buster, and from the log I can't (yet) determine why it'd fail on POWER other than it's possible the native compile path is different and somehow broken on Buster.
I'd like to see this fixed, but I'm not 100% sure when I'd have time to get a new environment set up and test build, which is why I'd like to see the full log from scratch build. Maybe it's something simple (missing package?)