ARM64: Treat alignment faults as bus errors

Summary:
ARM64 currently treats all data abort exceptions as page faults. This
can cause infinite loops on non-page fault faults, such as alignment faults.

Since kernel-side alignment faults should be avoided, this adds support directly
to the el0 fault handler, instead of the data_abort() handler.

Test Plan: Tested on rpi3, with a misaligned ldm test.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D22133