[Runtime] use pthread_getaffinity_np to determine default worker count

Merged Florian Fischer requested to merge aj46ezos/emper:improve-default-worker-count into master

The value reported by std::thread::hardware_concurrency() can be more than the actually available CPUs for example in a container or qemu.
pthread_getaffinity_np fills a CPU set with all CPU a thread can be scheduled on.

Fixes #19 (closed).

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