JIT: implement timer support in mtspr

Faster, of course, since we avoid the interpreter, but also means we can
get more a more accurate timer in long blocks by adding the offset from the
start of the block to the retrieved timer. I don't know if this will actually
fix any issues, but it's more correct and a nearly-free improvement.

Source/Core/Core/HW/SystemTimers.h

@@ -9,6 +9,31 @@
 namespace SystemTimers
 {
 
+/*
+GameCube                   MHz
+flipper <-> ARAM bus:      81 (DSP)
+gekko <-> flipper bus:     162
+flipper <-> 1T-SRAM bus:   324
+gekko:                     486
+
+These contain some guesses:
+Wii                             MHz
+hollywood <-> GDDR3 RAM bus:    ??? no idea really
+broadway <-> hollywood bus:     243
+hollywood <-> 1T-SRAM bus:      486
+broadway:                       729
+*/
+// Ratio of TB and Decrementer to clock cycles.
+// TB clk is 1/4 of BUS clk. And it seems BUS clk is really 1/3 of CPU clk.
+// So, ratio is 1 / (1/4 * 1/3 = 1/12) = 12.
+// note: ZWW is ok and faster with TIMER_RATIO=8 though.
+// !!! POSSIBLE STABLE PERF BOOST HACK THERE !!!
+
+enum
+{
+	TIMER_RATIO = 12
+};
+
 u32 GetTicksPerSecond();
 void PreInit();
 void Init();