[POWERPC] Add IRQSTACKS support on ppc32

This makes it possible to use separate stacks for hard and soft IRQs
on 32-bit powerpc as well as on 64-bit.  The code for 32-bit is just
the 32-bit analog of the 64-bit code.

* Added allocation and initialization of the irq stacks.  We limit the
  stacks to be in lowmem for ppc32.
* Implemented ppc32 versions of call_do_softirq() and call_handle_irq()
  to switch the stack pointers
* Reworked how we do stack overflow detection.  We now keep around the
  limit of the stack in the thread_struct and compare against the limit
  to see if we've overflowed.  We can now use this on ppc64 if desired.

[ [email protected]: Fixed bug on 6xx where we need to reload r9 with the
  thread_info pointer. ]

Signed-off-by: Kumar Gala <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>

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