PowerPC: Add functions to read/write the full timebase value

Allows us to get rid of a silly pointer cast and deduplicate some code
from the front-end when it comes to reading the value.

Source/Core/Core/PowerPC/PowerPC.h

@@ -178,6 +178,9 @@ void RunLoop();
 u32 CompactCR();
 void ExpandCR(u32 cr);
 
+u64 ReadFullTimeBaseValue();
+void WriteFullTimeBaseValue(u64 value);
+
 void UpdatePerformanceMonitor(u32 cycles, u32 num_load_stores, u32 num_fp_inst);
 
 // Easy register access macros.