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/core/core.h

@@ -47,6 +47,10 @@ namespace VideoCore {
 class RendererBase;
 } // namespace VideoCore
 
+namespace Core::Timing {
+class CoreTiming;
+}
+
 namespace Core {
 
 class ARM_Interface;
@@ -205,6 +209,12 @@ public:
     /// Provides a constant pointer to the current process.
     const Kernel::Process* CurrentProcess() const;
 
+    /// Provides a reference to the core timing instance.
+    Timing::CoreTiming& CoreTiming();
+
+    /// Provides a constant reference to the core timing instance.
+    const Timing::CoreTiming& CoreTiming() const;
+
     /// Provides a reference to the kernel instance.
     Kernel::KernelCore& Kernel();