opencrypto: Respect alignment constraints in xor_and_encrypt()

Copy operands to an aligned buffer before performing operations which
require alignment. Otherwise it's possible for this code to trigger an
alignment fault on armv7.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D41211