[LibFuzzer] Fix ``NumberOfCpuCores()`` on Mac OSX.

The ``nprocs`` command does not exist under Mac OSX so use
``sysctl`` instead on that platform.

Whilst I'm here

* Use ``pclose()`` instead of ``fclose()`` which the ``popen()``
  documentation says should be used.
* Check for errors that were previously unhandled.

Differential Revision: http://reviews.llvm.org/D20409

llvm-svn: 270172