sys/pm: Correctly access pm_blocker
sys/pm: Correctly access pm_blocker

Replace `volatile` access to pm_blocker by guarding the accesses with
`irq_disable()` ... `irq_restore()`.

`volatile` does only guarantee that no compiler optimizations are performed on
a variable access, but does not provide atomic access. E.g. on systems with
a memory bus of less than 32 bit, the access to pm_blocker cannot be done
with a single CPU instruction. Thus, resorting to disabling IRQs is the easiest
and most portable way to actually achieve atomic access.

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