Core: Bring over some file related Switch changes.

Reducing the distance from the Switch port code.

Core/System.cpp

@@ -609,7 +609,7 @@ void InitSysDirectories() {
 		INFO_LOG(COMMON, "Memstick directory not present, creating at '%s'", g_Config.memStickDirectory.c_str());
 	}
 
-	const std::string testFile = g_Config.memStickDirectory + "/_writable_test.$$$";
+	const std::string testFile = g_Config.memStickDirectory + "_writable_test.$$$";
 
 	// If any directory is read-only, fall back to the Documents directory.
 	// We're screwed anyway if we can't write to Documents, or can't detect it.