Simplify implementation of GetProcessorCount().

The current implementation is unnecessarily complex, because:

- The BSD derived systems implement sysconf(_SC_NPROCESSORS_ONLN)
  in terms of sysctl({CTL_HW,HW_NCPU}).
- get_nprocs() is a GNU extension, and glibc implements
  sysconf(_SC_NPROCESSORS_ONLN) in terms of get_nprocs().
Merge pull request #647 from bmeurer/SimplifyGetProcessorCount

Simplify implementation of GetProcessorCount().