Add irq_work support for LoongArch via self IPIs. This make it possible
to run works in hardware interrupt context, which is a prerequisite for
NOHZ_FULL.

Implement:
 - arch_irq_work_raise()
 - arch_irq_work_has_interrupt()
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>