cpu/cortexm_common: flush pipeline before disabling interrupts in idle
cpu/cortexm_common: flush pipeline before disabling interrupts in idle

When enabling & disabling interrupts back-to-back pending interrupts
are not serviced on Cortex-M23/M33.

Flush the pipeline to give interrupts a chance of executing in `sched_arch_idle()`.

This fixes `no_idle_thread` on Cortex-M23.

RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT