arm: Convert to GENERIC_IRQ_MULTI_HANDLER

This converts the ARM port to use the recently added
GENERIC_IRQ_MULTI_HANDLER, which is essentially just a copy of ARM's
existhing MULTI_IRQ_HANDLER.  The only changes are:

* handle_arch_irq is now defined in a generic C file instead of an
  arm-specific assembly file.
* handle_arch_irq is not marked as __ro_after_init.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>