xen/arm: Init traps very early

xen/arm: Init traps very early

The function init_traps sets up the handler taken when Xen hits a
BUG_ON/ASSERT.

If an error happen before init_traps is called, we lose the backtrace.

When a trap is taken from HYP mode, we need to:
    - get the processor ID, percpu has to be initialized and the
      smp_processor_id correctly set.
    - initialize current, even though it's not used for the time being. Move
      the debug sanity earlier.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Tested-by: Naresh Bhat <naresh.bhat@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fixed typos and rewrapped commit log ]