arm64: add undefined instruction handler hooks

Add undefined instruction handler hooks similar to the system in the
arm archetecture. One difference is that hooks can only be added at
boot time and they can never be removed. This removes the need for
the spinlock in the handler.

Change-Id: I4684937f5209ca2a64ee63947bb2ab6411ae14f7
Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-on: http://git-master/r/361736
Reviewed-on: http://git-master/r/365059


Reviewed-by: Richard Wiley <rwiley@nvidia.com>
Tested-by: Oskari Jaaskelainen <oskarij@nvidia.com>