MINOR: cpu-topo: add detection of online CPUs on FreeBSD

MINOR: cpu-topo: add detection of online CPUs on FreeBSD

On FreeBSD we can detect online CPUs at least by doing the bitwise-OR of
the CPUs of all domains, so we're using this and adding this detection
to ha_cpuset_detect_online(). If we find simpler later, we can always
rework it, but it's reasonably inexpensive since we only check existing
domains.