Change Priority_Control to 64-bit for proper EDF support

[Sebastian Huber]

A 32-bit Priority_Control limits the uptime to 49 days with a 1ms clock
tick in case the EDF scheduler is used.  Increase it to 64-bit to enable
proper operation of the EDF scheduler,

I temporarily changed confdefs.h to use the EDF scheduler as default.  All
tests passed using the EDF scheduler as the default scheduler.  So, the EDF
scheduler is now a viable alternative to the standard fixed-priority scheduler.