aarch64 - Emit note if crc instructions available and not used

[swalk-cavium]

The ability to use the ARMv8 crc instructions was added by PR#7556. They
are enabled only if an option is added to the cmake command line.

This makes a minor change to the HPHPCompiler.cmake file to emit a note
if the build host supports the crc instruction and it has not been selected.

[swalk-cavium]

@mxw - Hi Max, Can you inspect?

[swalk-cavium]

@jim-saxman, @dave-estes, @cmuellnerTheobromaSystems - Any comments? It didn't seem right to change options automatically on the user, so I figured a note would be sufficient.

[jim-saxman]

@swalk-cavium Works for me with our standard CMake command line. However, we're using the -DAARCH64_TARGET_CPU=qdf24xx in conjunction with the -DENABLE_AARCH64_CRC=1. Can I ask what your cmake looks like?

[swalk-cavium]

@jim-saxman - I had been letting it default. That's when I noticed the one regression test slowing down.