arm64: Use the new GENERIC_IRQ_MULTI_HANDLER

It appears arm64 copied arm's GENERIC_IRQ_MULTI_HANDLER code, but made
it unconditional.  I wanted to make this generic so it could be used by
the RISC-V port.  This patch converts the arm64 code to use the new
generic code, which simply consists of deleting the arm64 code and
setting MULTI_IRQ_HANDLER instead.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>