arm64: Raise a SIGBUS on a user external abort

When userspace triggers an external abort allow it to handle the abort
by raising a SIGBUS.

Reviewed by: markj, kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42018

(cherry picked from commit a69450102ff1fe1466077a021a16bb265274d3a2)