N64 loading improvements (#804)

* add n64 type

* use a custom loader for N64

This loader:
- converts little-endian and byte-swapped ROMs
- auto-detects ROM properties, this requires a N64-database.txt file in
  the N64 games folder

menu.cpp

@@ -2407,6 +2407,10 @@ void HandleUI(void)
 					neocd_set_en(0);
 					neogeo_romset_tx(selPath, 0);
 				}
+				else if (is_n64())
+				{
+					if (!n64_rom_tx(selPath, idx)) Info("failed to load ROM");
+				}
 				else
 				{
 					if (is_pce())