Gekko: Make register constructors explicit where applicable

Prevents implicit conversions to types and requires explicitly
specifying them in order to construct instances of them. Given these are
used within emulation code directly, being explicit is always better
than implicit.

Source/Core/Core/PowerPC/PowerPC.h

@@ -303,7 +303,7 @@ inline UReg_XER GetXER()
   xer |= PowerPC::ppcState.xer_stringctrl;
   xer |= PowerPC::ppcState.xer_ca << XER_CA_SHIFT;
   xer |= PowerPC::ppcState.xer_so_ov << XER_OV_SHIFT;
-  return xer;
+  return UReg_XER{xer};
 }
 
 inline void SetXER(UReg_XER new_xer)