cmake: use `nproc` if _num_cores is zero #259

The previous workaround set _num_cores to 1 if cmake thought NUMBER_OF_LOGICAL_CORES was zero. That worked fine, but it'd be much better to take advantage of all possible cores. This version runs `nproc` which seems to work just fine under qemu-aarch64 and gives us the actual number of CPUs. Fixes: b666926 Signed-off-by: Tim Serong <[email protected]>