armv8crypto: add AES-XTS support

A straightforward(ish) port from aesni(4). This implementation does not
perform loop unrolling on the input blocks, so this is left as a future
performance improvement.

Submitted by: Greg V <greg AT unrelenting.technology>
Looks good: jhb, jmg
Tested by: mhorne
Differential Revision: https://reviews.freebsd.org/D21017