Build: More consistently avoid _M_ defines.

We use PPSSPP_ARCH in several places already, this makes it more complete.

Core/Config.cpp

@@ -988,7 +988,7 @@ static ConfigSetting networkSettings[] = {
 
 static int DefaultPSPModel() {
 	// TODO: Can probably default this on, but not sure about its memory differences.
-#if !defined(_M_X64) && !defined(_WIN32)
+#if !PPSSPP_ARCH(AMD64) && !defined(_WIN32)
 	return PSP_MODEL_FAT;
 #else
 	return PSP_MODEL_SLIM;