arm-v7m: disable irqs while rescheduling
arm-v7m: disable irqs while rescheduling

Interrupts might preempt the pendsv handler. If the pendsv handler was
rescheduling and that the interrupt that preempted tries to modify the runqueue,
then we will have unprotected concurrent access to the same data, and who
knows what will happen?

Signed-off-by: Fabien Parent <2a93cde040adee823a667eed2882d05f12a84573@gmail.com>

gregkh/phabos,gregkh/phabos,fabo-corp/phabos,fabo-corp/phabos,mtitinger/phabos,mtitinger/phabos