Initial Intel SSE4 crc32c implementation.

Signed-off-by: Mark Nelson <mark.nelson@inktank.com>
common: autoselect crc32c based on cpu features

If the CPu supposts SSE4.2, use the crc32c instructions.  Use the magic
incantation from who knows where to do this.  __builtin_cpu_supports()
is a nicer way to do it, but that is new in gcc 4.8.

Avoid static globals; they are bad.  Sadly that means we redetect the CPU
feature on every call. I assume that is reasonably efficient...

Signed-off-by: Sage Weil <sage@inktank.com>