Use the vector polynomial multiply-sum instructions in POWER8 to
speed up crc32c.

This is just over 41x faster than the slice-by-8 method that it
replaces. Measurements on a 4.1 GHz POWER8 show it sustaining
52 GiB/sec.

A simple btrfs write performance test:

    dd if=/dev/zero of=/mnt/tmpfile bs=1M count=4096
    sync

is over 3.7x faster.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>