Gekko: Centralize bitmasking of the FPSCR within UReg_FPSCR

Rather than introduce this handling in every system instruction that modifies
the FPSCR directly, we can instead just handle it within the data structure
instead, which avoids duplicating mask handling across instructions.

This also allows handling proper masking from the debugger register
windows themselves without duplicating masking behavior there either.

Source/Core/DolphinWX/Debugger/RegisterView.cpp

@@ -124,7 +124,7 @@ void SetSpecialRegValue(int reg, u32 value)
     PowerPC::SetXER(UReg_XER(value));
     break;
   case 5:
-    PowerPC::ppcState.fpscr.Hex = value;
+    PowerPC::ppcState.fpscr = value;
     break;
   case 6:
     PowerPC::ppcState.msr.Hex = value;