x86_64: Fix atomic functions
kernel: Make sure mutex::holder is set to a valid value
scheduler: Improve power saving mode

 * Remove possibility to temporarily disable small task packing.
 * When small task packing target gets overloaded continue packing
   threads on another core, but avoid migrating the already packed
   ones.

Scheduler still tends to needlessly migrate threads to another cores
when under heavier load, but it is now much better than before.