[x64] Change the parameters to host_to_guest_thunk

Shuffle some code around in x64_backend.cc
Add GetNativeParam to avoid hardcoding parameters

src/xenia/cpu/backend/x64/x64_emitter.h

@@ -187,6 +187,8 @@ class X64Emitter : public Xbyak::CodeGenerator {
   void CallNativeSafe(void* fn);
   void SetReturnAddress(uint64_t value);
 
+  Xbyak::Reg64 GetNativeParam(uint32_t param);
+
   Xbyak::Reg64 GetContextReg();
   Xbyak::Reg64 GetMembaseReg();
   void ReloadContext();