Add support for the Google xxhash checksumming function

It's a lot faster than the other software defined ones, so will
be a useful alternative for the cases where crc32c-intel isn't
available.

Signed-off-by: Jens Axboe <axboe@fb.com>
Add option group/category to allrandrepeat

Signed-off-by: Jens Axboe <axboe@fb.com>
xxhash: dos2unix'ize

Signed-off-by: Jens Axboe <axboe@fb.com>