Make sure that RA on top of interrupt stack is an address of ret_from_irq,
so that dump_stack etc. can trace info interrupted context.

Also this patch fixes except_vec_vi_handler and __smtc_ipi_vector which
seems broken.
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>