For llvm's internal function which retrieves the number of available
"hardware threads", use cpuset_getaffinity(2) on FreeBSD, so it will
honor processor sets configured by the cpuset(1) command.

This should make it possible to avoid e.g. lld creating a huge number of
threads on a machine with many cores, even for linking simple programs.

This will also be submitted upstream.

Submitted by: mjg
MFC after: 1 week