Use LDREXW + CLREX + STREXW read and clear CDC break value

Instead of NVIC_DisableIRQ + NVIC_EnableIRQ, as it’s a bit awkward to
check if a particular NVIC IRQ is enabled on ARM. This is similar
approach to Serial_::accept.

Author: sandeepmistry

hardware/arduino/sam/cores/arduino/USB/CDC.cpp

@@ -309,17 +309,18 @@ Serial_::operator bool()
 }
 
 int32_t Serial_::readBreak() {
-	// disable the USB OTG interrupt,
-	// to avoid clearing a breakValue that might occur 
-	// while reading the current break value
-	NVIC_DisableIRQ((IRQn_Type) ID_UOTGHS);
-
-	int32_t ret = breakValue;
-
-	breakValue = -1;
-
-	// re-eable the USB OTG interupt
-	NVIC_EnableIRQ((IRQn_Type) ID_UOTGHS);
+	// read the breakValue, using exclusive read
+	int32_t ret = __LDREXW((volatile uint32_t*)&breakValue);
+
+	if (ret == -1) {
+		// no new break value, clear exclusive access record
+		__CLREX();
+	} else {
+		// only set it to -1 (no break),
+		// if a new value has not been written in the ISR,
+		// using exclusive write
+		__STREXW(-1, (volatile uint32_t*)&breakValue);
+	}
 
 	return ret;
 }