Avoid reseved identifiers like _SP, etc.

R_SP is not that bad.

Common/ArmEmitter.cpp

@@ -395,7 +395,7 @@ void ARMXEmitter::MOVI2R(ARMReg reg, u32 val, bool optimize)
 			// Use literal pool for ARMv6.
 			// Disabled for now as it is crashing since Vertex Decoder JIT
 //			AddNewLit(val);
-//			LDR(reg, _PC); // To be backpatched later
+//			LDR(reg, R_PC); // To be backpatched later
 		}
 #endif
 	}