Add processor topology calculation implementation for Darwin/OSX targets.

Add processor topology calculation implementation for Darwin/OSX targets.

The implementation for bootstrapping SWR on Darwin targets is based on the Linux version.
Instead of reading the output of /proc/cpuinfo, sysctlbyname is used to determine the
physical identifiers, processor identifiers, core counts and thread-processor affinities.

With this patch, it is possible to use SWR as an alternate renderer on OSX to softpipe and
llvmpipe.

Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>