Everywhere: Make tracking cpu usage independent from system ticks

This switches tracking CPU usage to more accurately measure time in
user and kernel land using either the TSC or another time source.
This will also come in handy when implementing a tickless kernel mode.

Kernel/Time/TimeManagement.h

@@ -80,6 +80,8 @@ private:
     void set_system_timer(HardwareTimerBase&);
     static void system_timer_tick(const RegisterState&);
 
+    static u64 scheduling_current_time(bool);
+
     // Variables between m_update1 and m_update2 are synchronized
     Atomic<u32> m_update1 { 0 };
     u32 m_ticks_this_second { 0 };
@@ -91,6 +93,7 @@ private:
 
     u32 m_time_ticks_per_second { 0 }; // may be different from interrupts/second (e.g. hpet)
     bool m_can_query_precise_time { false };
+    bool m_updating_time { false }; // may only be accessed from the BSP!
 
     RefPtr<HardwareTimerBase> m_system_timer;
     RefPtr<HardwareTimerBase> m_time_keeper_timer;