xen/arm: Panic when we receive an unexpected trap

xen/arm: Panic when we receive an unexpected trap

The current implementation of do_unexpected_trap make Xen spin forever
on the current physical CPU. This may lead to stall guests VCPU and print
unhelpful message (RCU stall...).

Usually when Xen receives an unexpected trap, it means that something goes
wrong either in the hypervisor or in the CPU. In this case we should
directly panic to also stop the other CPUs.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>