common: add NEON crc32c implementation

Signed-off-by: Sage Weil <sage@inktank.com>
common/crc32c_intel_fast: fix compile-time #ifdef

This wasn't getting built in!

Signed-off-by: Sage Weil <sage@inktank.com>
common: unit test for crc32c

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