Finish the GENERIC_IRQ_MULTI_HANDLER conversion

[Palmer Dabbelt]

A while ago I sent a patch set that adds a GENERIC_IRQ_MULTI_HANDLER,
which is an exact copy of the existing IRQ_MULTI_HANDLER support in the
arm port, which is being used unconditionally by arm64 and openrisc.
GENERIC_IRQ_MULTI_HANDLER is currently being used by the RISC-V port.  I
managed to make a few mistakes in my original patch set and as a result
my conversion of the other architectures of GENERIC_IRQ_MULTI_HANDLER
was dropped.

This patch set finishes up my original patch set by converting arm,
arm64, and openrisc over to the new GENERIC_IRQ_MULTI_HANDLER support
and then removing MULTI_IRQ_HANDLER as it's obselete.

At the time I wrote this I gave it fairly extensive build testing, but
went I recently rebased it I just tested the full patch set on arm,
arm64, and openrisc defconfigs.

Various flavors of this patch set have bounced around a few times
before, but I'm calling this a whole new patch set as it builds on top
of what was merged.