genirq: Move non-irqdomain handle_domain_irq() handling into ARM's handle_IRQ()

Despite the name, handle_domain_irq() deals with non-irqdomain
handling for the sake of a handful of legacy ARM platforms.

Move such handling into ARM's handle_IRQ(), allowing for better
code generation for everyone else. This allows us get rid of
some complexity, and to rearrange the guards on the various helpers
in a more logical way.

Signed-off-by: Marc Zyngier <maz@kernel.org>