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/gpu.cpp

@@ -3,6 +3,7 @@
 // Refer to the license.txt file included.
 
 #include "common/assert.h"
+#include "core/core.h"
 #include "core/core_timing.h"
 #include "core/memory.h"
 #include "video_core/engines/fermi_2d.h"
@@ -283,7 +284,7 @@ void GPU::ProcessSemaphoreTriggerMethod() {
         block.sequence = regs.semaphore_sequence;
         // TODO(Kmather73): Generate a real GPU timestamp and write it here instead of
         // CoreTiming
-        block.timestamp = Core::Timing::GetTicks();
+        block.timestamp = Core::System::GetInstance().CoreTiming().GetTicks();
         Memory::WriteBlock(*address, &block, sizeof(block));
     } else {
         const auto address =