Hi Daniel,
the Fiasco.OC-specific implementation is indeed missing. It should go to 'base-foc/src/core/include/platform.h'. Apparently, we have used the 'affinity' function but not the 'num_cpus' function in our L4Linux support code. So the missing 'num_cpus' slipped through. Sorry about that. I have opened an issue:
https://github.com/genodelabs/genode/issues/578
Thanks for reporting!
Norman