[POWERPC] Make Book-E debug handling SMP safe

global_dbcr0 needs to be a per cpu set of save areas instead of a single
global on all processors.

Also, we switch to using DBCR0_IDM to determine if the user space app is
being debugged as its a more consistent way.  In the future we should
support features like hardware breakpoint and watchpoints which will
have DBCR0_IDM set but not necessarily DBCR0_IC (single step).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>