[Runtime] use pthread_getaffinity_np to determine default worker count

The value reported by std::thread::hardware_concurrency() can be more
than the available CPUs to the current process for example if EMPER runs
in a container or qemu.
GNU nproc(1) does the right thing and we can take inspiration from them.
The solution is to use pthread_getaffinity_np which fills a CPU set with
all CPU a pthread can be scheduled on.
We use this more precise CPU count if possible.