kernel/scheduler: Pass in system instance in constructor

Avoids directly relying on the global system instance and instead makes
an arbitrary system instance an explicit dependency on construction.

This also allows removing dependencies on some global accessor functions
as well.

src/core/core_cpu.h

@@ -15,6 +15,10 @@ namespace Kernel {
 class Scheduler;
 }
 
+namespace Core {
+class System;
+}
+
 namespace Core::Timing {
 class CoreTiming;
 }
@@ -45,8 +49,8 @@ private:
 
 class Cpu {
 public:
-    Cpu(Timing::CoreTiming& core_timing, ExclusiveMonitor& exclusive_monitor,
-        CpuBarrier& cpu_barrier, std::size_t core_index);
+    Cpu(System& system, ExclusiveMonitor& exclusive_monitor, CpuBarrier& cpu_barrier,
+        std::size_t core_index);
     ~Cpu();
 
     void RunLoop(bool tight_loop = true);