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_timing.h

@@ -148,6 +148,7 @@ public:
 
     class Timer {
     public:
+        Timer(double cpu_clock_scale);
         ~Timer();
 
         s64 GetMaxSliceLength() const;
@@ -190,10 +191,13 @@ public:
         s64 slice_length = MAX_SLICE_LENGTH;
         s64 downcount = MAX_SLICE_LENGTH;
         s64 executed_ticks = 0;
-        u64 idled_cycles;
+        u64 idled_cycles = 0;
+        // Stores a scaling for the internal clockspeed. Changing this number results in
+        // under/overclocking the guest cpu
+        double cpu_clock_scale = 1.0;
     };
 
-    explicit Timing(std::size_t num_cores);
+    explicit Timing(std::size_t num_cores, u32 cpu_clock_percentage);
 
     ~Timing(){};
 
@@ -220,6 +224,11 @@ public:
         global_timer += ticks;
     }
 
+    /**
+     * Updates the value of the cpu clock scaling to the new percentage.
+     */
+    void UpdateClockSpeed(u32 cpu_clock_percentage);
+
     std::chrono::microseconds GetGlobalTimeUs() const;
 
     std::shared_ptr<Timer> GetTimer(std::size_t cpu_id);
@@ -229,10 +238,14 @@ private:
 
     // unordered_map stores each element separately as a linked list node so pointers to
     // elements remain stable regardless of rehashes/resizing.
-    std::unordered_map<std::string, TimingEventType> event_types;
+    std::unordered_map<std::string, TimingEventType> event_types = {};
 
     std::vector<std::shared_ptr<Timer>> timers;
     std::shared_ptr<Timer> current_timer;
+
+    // Stores a scaling for the internal clockspeed. Changing this number results in
+    // under/overclocking the guest cpu
+    double cpu_clock_scale = 1.0;
 };
 
 } // namespace Core