[POWERPC] Rework EXC_LEVEL_EXCEPTION_PROLOG code

* Cleanup the code a bit my allocating an INT_FRAME on our exception
  stack there by make references go from GPR11-INT_FRAME_SIZE(r8) to
  just GPR11(r8)
* simplify {lvl}_transfer_to_handler code by moving the copying of the
  temp registers we use if we come from user space into the PROLOG
* If the exception came from kernel mode copy thread_info flags,
  preempt, and task pointer from the process thread_info.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Paul Mackerras <paulus@samba.org>