Teach DTrace that unaligned accesses are OK on aarch64, not just x86.
Reviewed by: andrew
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D29369
(cherry picked from commit fb581531c1a0e6f1bf5392a2e97ed39d21d6e1fd)
(cherry picked from commit ba08ba5226ff8551ea3926ce49b8604b71c9cabf)