cmd: exception: arm64: fix undefined, add faults

The arm64 version of the exception command was just defining the
undefined exception, but actually copied the AArch32 instruction.

Replace that with an encoding that is guaranteed to be and stay
undefined. Also add instructions to trigger unaligned access faults and
a breakpoint.
This brings ARM64 on par with ARM(32) for the exception command.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>