Eliminate irq_dispatch.S.  Move the data items it contained into arm/intr.c
and the functionality it provided into arm/exception.S.  Rename the main
irq handling routine from arm_handler_execute() to arm_irq_handler() to
make it more congruent with how other exception handlers are named, and
also update its signature to reflect what has long been reality: it is
passed just a trapframe pointer, no interrupt number argument.