core_timing: Convert core timing into a class

Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.

Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.

src/video_core/renderer_opengl/renderer_opengl.cpp

@@ -137,7 +137,7 @@ void RendererOpenGL::SwapBuffers(
 
     render_window.PollEvents();
 
-    system.FrameLimiter().DoFrameLimiting(Core::Timing::GetGlobalTimeUs());
+    system.FrameLimiter().DoFrameLimiting(system.CoreTiming().GetGlobalTimeUs());
     system.GetPerfStats().BeginSystemFrame();
 
     // Restore the rasterizer state