Use constexpr constructors for all Serial classes

teensy3/HardwareSerial.h

 class HardwareSerial : public Stream
 {
 public:
+	constexpr HardwareSerial() {}
 	virtual void begin(uint32_t baud) { serial_begin(BAUD2DIV(baud)); }
 	virtual void begin(uint32_t baud, uint32_t format) {
 					  serial_begin(BAUD2DIV(baud));
 class HardwareSerial2 : public HardwareSerial
 {
 public:
+	constexpr HardwareSerial2() {}
 	virtual void begin(uint32_t baud) { serial2_begin(BAUD2DIV2(baud)); }
 	virtual void begin(uint32_t baud, uint32_t format) {
 					  serial2_begin(BAUD2DIV2(baud));
 class HardwareSerial3 : public HardwareSerial
 {
 public:
+	constexpr HardwareSerial3() {}
 	virtual void begin(uint32_t baud) { serial3_begin(BAUD2DIV3(baud)); }
 	virtual void begin(uint32_t baud, uint32_t format) {
 					  serial3_begin(BAUD2DIV3(baud));
 class HardwareSerial4 : public HardwareSerial
 {
 public:
+	constexpr HardwareSerial4() {}
 	virtual void begin(uint32_t baud) { serial4_begin(BAUD2DIV3(baud)); }
 	virtual void begin(uint32_t baud, uint32_t format) {
 					  serial4_begin(BAUD2DIV3(baud));
 class HardwareSerial5 : public HardwareSerial
 {
 public:
+	constexpr HardwareSerial5() {}
 	virtual void begin(uint32_t baud) { serial5_begin(BAUD2DIV3(baud)); }
 	virtual void begin(uint32_t baud, uint32_t format) {
 					  serial5_begin(BAUD2DIV3(baud));
 class HardwareSerial6 : public HardwareSerial
 {
 public:
+	constexpr HardwareSerial6() {}
 #if defined(__MK66FX1M0__)	// For LPUART just pass baud straight in. 
 	virtual void begin(uint32_t baud) { serial6_begin(baud); }
 	virtual void begin(uint32_t baud, uint32_t format) {

teensy3/usb_seremu.h

 class usb_seremu_class : public Stream
 {
 public:
+	constexpr usb_seremu_class() {}
         void begin(long) { /* TODO: call a function that tries to wait for enumeration */ };
         void end() { /* TODO: flush output and shut down USB port */ };
         virtual int available() { return usb_seremu_available(); }
 class usb_seremu_class : public Stream
 {
 public:
+	constexpr usb_seremu_class() {}
 	void begin(long) { };
 	void end() { };
 	virtual int available() { return 0; }

teensy3/usb_serial.h

 class usb_serial_class : public Stream
 {
 public:
+	constexpr usb_serial_class() {}
         void begin(long) {
 		uint32_t millis_begin = systick_millis_count;
 		while (!(*this)) {
 class usb_serial_class : public Stream
 {
 public:
+	constexpr usb_serial_class() {}
         void begin(long) { };
         void end() { };
         virtual int available() { return 0; }