arm64: Mask non-debug exceptions when single stepping

When an exception is pending when single stepping we may execute the
handler for that exception rather than the single step handler. This
could cause the scheduler to fire to run a new thread. This will mean
we single step to a new thread causing unexpected results.

Handle this by masking non-debug exceptions. This will cause issues
when stepping over instructions that access the DAIF values so future
work is needed to handle these cases, but for most code this now works
as expected.

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44350