Fix flush hanging issue

Make write to UDR and clearing of TXC bit in flush() atomic
to avoid race condition.

Fixes #3745 (second different issue introduced later but discussed
in the same issue)

cores/arduino/HardwareSerial.cpp

@@ -226,8 +226,18 @@ size_t HardwareSerial::write(uint8_t c)
   // significantly improve the effective datarate at high (>
   // 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
   if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
-    *_udr = c;
-    *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);
+    // If TXC is cleared before writing UDR and the previous byte
+    // completes before writing to UDR, TXC will be set but a byte
+    // is still being transmitted causing flush() to return too soon.
+    // So writing UDR must happen first.
+    // Writing UDR and clearing TC must be done atomically, otherwise
+    // interrupts might delay the TXC clear so the byte written to UDR
+    // is transmitted (setting TXC) before clearing TXC. Then TXC will
+    // be cleared when no bytes are left, causing flush() to hang
+    ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+      *_udr = c;
+      *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);
+    }
     return 1;
   }
   tx_buffer_index_t i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;