arm64: Use a switch to decide when to enable debug

Use a switch statement to decide which exceptions we need to call
dbg_enable for. This simplifies adding more esceptions to the list in
the future.

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44352

(cherry picked from commit d93b3a65f7697630ca1dd2635fc976f3ef36b602)