arm: restore kernel on setup failure

arm: restore kernel on setup failure

This patch implements two cases:
- When an error occurs before setting up EL2, there is nothing much
  to do except restore the linux registers stored in the per_cpu
  datas.
- When it happens after EL2 setup, arch_cpu_restore copies the saved
  registers on the stack, and continues into arch_shutdown_self

When it happens during the MMU setup, chances of recovering a clean
state are pretty thin anyway. The bootstrap vectors could be used to
catch and dump a minimal context (which would require a raw_printk
implementation), but we cowardly ignore this case for the moment.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: fix memcpy size in cpu_return_el1]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>