Core, VideoCommon: Fix crash at shutdown due to destructor ordering

Previously, PerformanceTracker registered a callback to be updated on
emulation state changes. PerformanceTrackers live in a global variable
(g_perf_metrics) within libvideocommon. The callback was stored in a
global variable in libcore. This created a race condition at shutdown
between these libraries, when the PerfTracker's destructor tried to
unregister the callback.
Notify the PerfTracker directly from libcore, without callbacks, since
Core.cpp already references g_perf_metrics explicitly. Also rename
Core::CallOnStateChangedCallbacks to NotifyStateChanged to better
reflect what it's doing.

Source/Core/Core/PowerPC/GDBStub.cpp

@@ -868,7 +868,7 @@ static void Step()
 {
   auto& system = Core::System::GetInstance();
   system.GetCPU().SetStepping(true);
-  Core::CallOnStateChangedCallbacks(Core::State::Paused);
+  Core::NotifyStateChanged(Core::State::Paused);
 }
 
 static bool AddBreakpoint(BreakpointType type, u32 addr, u32 len)