Gekko: Make UReg_FPSCR's single argument constructor explicit

Prevent implicit conversions to UReg_FPSCR. Given the semantics of a
random magic value and the FPSCR are different, make explicit
conversions a requirement to signify intent.

Source/Core/Core/PowerPC/PowerPC.cpp

@@ -128,7 +128,7 @@ static void ResetRegisters()
   ppcState.spr[SPR_ECID_M] = 0x1840c00d;
   ppcState.spr[SPR_ECID_L] = 0x82bb08e8;
 
-  ppcState.fpscr = 0;
+  ppcState.fpscr.Hex = 0;
   ppcState.pc = 0;
   ppcState.npc = 0;
   ppcState.Exceptions = 0;