common/crc32c_neon: disable for now

The NEON code only works with an initial crc vector of 0xffffffff, and even
when that is true does not seem to be faster than the sctp code.  Stick
with that for now, but keep all the build and runtime arch stuff in place
for when we *do* get a faster implementation.

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