Teensy 4.x - SerialUSB1 and SerialUSB2 objects created.

The actual objects were not created like they were on the T3.x side, so just copied it over from there.

teensy4/usb_inst.cpp

 #endif
 #endif
 
+#ifdef CDC2_DATA_INTERFACE
+#ifdef CDC2_STATUS_INTERFACE
+usb_serial2_class SerialUSB1;
+#endif
+#endif
+
+#ifdef CDC3_DATA_INTERFACE
+#ifdef CDC3_STATUS_INTERFACE
+usb_serial3_class SerialUSB2;
+#endif
+#endif
+
 #ifdef MIDI_INTERFACE
 usb_midi_class usbMIDI;
 #endif