Add CPU Clock Frequency slider

This slider affects the number of cycles that the guest cpu emulation
reports that have passed since the last time slice. This option scales
the result returned by a percentage that the user selects. In some games
underclocking the CPU can give a major speedup. Exposing this as an
option will give users something to toy with for performance, while also
potentially enhancing games that experience lag on the real console

src/core/core.cpp

@@ -256,7 +256,7 @@ System::ResultStatus System::Init(Frontend::EmuWindow& emu_window, u32 system_mo
 
     memory = std::make_unique<Memory::MemorySystem>();
 
-    timing = std::make_unique<Timing>(num_cores);
+    timing = std::make_unique<Timing>(num_cores, Settings::values.cpu_clock_percentage);
 
     kernel = std::make_unique<Kernel::KernelSystem>(
         *memory, *timing, [this] { PrepareReschedule(); }, system_mode, num_cores);