cpu_count returns wrong results in the case of dynamically enabled CPU cores

[KOLANICH]

Some systems allow to disable CPU cores to save power. On these systems cpu_count usually returns the count of cores less than the available one, for example on 4 core cpu it usually returns 2 (unless something heavy is not started in background or the cores are not manually brought up).

[giampaolo]

See #1122
Closing this one as duplicate.