Changes for powerpc in git commit dec3f8ca assume power8 VSX
extensions for faster crc32 support, but earlier processors
(e.g. power5) don't have them.  On unexpected powerpc chips,
we will not use an optimized crc32 implementation.  CMake does
not provide a facility to detect processor features or in the
case of powerpc, flavors of the chip.  Create a place where we
can detect such flavors and caveat as necessary.