[MIPS] Provide empty irq_enable_hazard definition for legacy and R1 cores.

Following a strict interpretation the empty definition of irq_enable_hazard
has always been a bug - but an intentional one because it didn't bite.
This has now changed, for uniprocessor kernels mm/slab.c:do_drain()

[...]
        on_each_cpu(do_drain, cachep, 1, 1);
        check_irq_on();
[...]

may be compiled into a mtc0 c0_status; mfc0 c0_status sequence resulting
in a back-to-back hazard.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>