Add missing exception number to EL0 sync. abort on ARM64

When doing a data abort from userland it is possible to get
more than one data abort inside the same exception level.
Add an appropriate exception number to allow nesting of
data_abort handler for EL0.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3276