Disable iOS CPU Frequency readout (#335)

* Add macro definition for iOS

Add an additional macro definition for iOS.
iOS is defined as a Mac OSX invariant in the TargetConditionals include,
thus we treat it as a subset of OSX within the defines.

* Skip error for hw.cpufrequency on iOS

hw.cpufrequency is not available on iOS devices. As there is no way to reliably
retrieve the CPU frequency on iOS we are printing out a warning
that we were unable to detect the CPU frequency and set it to 0.

This only disables cpu frequency readouts on actual physical iOS devices.
Running this code on the simulator still gives the cpu architecture of
the host computer as the simulator passes down the sysctl calls to OSX.