Revert "scheduler: SCHEDULER_EXIT_FUNCTION() is not needed"

This reverts commit 667617ad043a4587d8d366d5192d9ad291cfa37a.

Scheduler profiler uses CPU local data to store function information, hence
arch_thread_context_switch() usually is not a problem. However, when
we switch to a new thread we end up scheduler_new_thread_entry() instead
of scheduler_reschedule() what may corrupt data collected by the profiler.

scheduler: SCHEDULER_EXIT_FUNCTION() is not needed

scheduler: profiler: Use nanosecond as a main time unit

scheduler: Ignore time used by the profiler implementation

scheduler: Add scheduler profiler

A bit hackish implementation of a profiler for the scheduler.
SCHEDULER_ENTER_FUNCTION at the begining of each function aren't nice and
usage of __PRETTY_FUNCTION__ isn't any better (both gcc and clang support
it though), but it was quick to implement and doesn't lose information
on inlined functions. It's just a tool, not an integral part of the kernal
anyway.

Revert "scheduler: SCHEDULER_EXIT_FUNCTION() is not needed"

This reverts commit 667617ad043a4587d8d366d5192d9ad291cfa37a.

Scheduler profiler uses CPU local data to store function information, hence
arch_thread_context_switch() usually is not a problem. However, when
we switch to a new thread we end up scheduler_new_thread_entry() instead
of scheduler_reschedule() what may corrupt data collected by the profiler.

scheduler: SCHEDULER_EXIT_FUNCTION() is not needed

scheduler: profiler: Use nanosecond as a main time unit

scheduler: Ignore time used by the profiler implementation

scheduler: Add scheduler profiler

A bit hackish implementation of a profiler for the scheduler.
SCHEDULER_ENTER_FUNCTION at the begining of each function aren't nice and
usage of __PRETTY_FUNCTION__ isn't any better (both gcc and clang support
it though), but it was quick to implement and doesn't lose information
on inlined functions. It's just a tool, not an integral part of the kernal
anyway.