The culprit appeared to be the config option for a "static link toolchain" which I enabled. This is quite logical for a bare metal target, but your config patch has a comment it doesn't always work, so I tried it without and that did the trick.
Next step is to find which of the options changes are really required to make the whole stuff work, additional to the ones already there in the Philipov repo and patch the config with it. I have some more specific options (like enabling floating point in newlib) that imho should not be in the generic config and I'll keep them in a local patch.