[CMake] Detect 32 bit build on a 64 bit system

[James Bigler]

Is it possible to detect a 32 bit build on a 64 bit system.

I know I can look at the CMAKE_SIZEOF_VOIDP to determine the bitness
of the build, but that can't tell me if I'm on a 64 bit linux system.
Is there another mechanism?

Thanks,
James