powerpc/irq: Move #ifdef'ed body of do_IRQ() into a separate function

Rather than a giant ifdef in the body of do_IRQ(), including a
dangling else, move the irq stack logic into a separate routine and
do the ifdef there.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>