machine/esp32s3,esp32c3: add txStalled flag to skip USB serial spin when no host

When no USB host is reading, flushAndWait() spins 50K iterations per
FIFO-full event. With putchar calling WriteByte per byte, the cumulative
delay starves I2C and other peripherals, freezing displays.

Add a txStalled flag: the first FIFO-full triggers one flushAndWait
attempt. If it fails (no host), txStalled is set and all subsequent
writes return immediately with no spin — just a register read and a
bool check. When a host reconnects, SERIAL_IN_EP_DATA_FREE goes back
to 1, bypassing the stall path and clearing the flag automatically.

src/runtime/runtime_esp32c3.go

@@ -151,6 +151,7 @@ func initTimerInterrupt() {
 // sleepTicks spins until the given number of ticks have elapsed, using the
 // TIMG0 alarm interrupt to avoid busy-waiting for the entire duration.
 func sleepTicks(d timeUnit) {
+	machine.FlushSerial()
 	target := ticks() + d
 	for ticks() < target {
 		// Set the alarm to fire at the target tick count (or as close

src/runtime/runtime_esp32sx.go

@@ -110,6 +110,7 @@ func initTimerInterrupt() {
 // sleepTicks spins until the given number of ticks have elapsed, using the
 // TIMG0 alarm interrupt to avoid busy-waiting for the entire duration.
 func sleepTicks(d timeUnit) {
+	machine.FlushSerial()
 	target := ticks() + d
 	for ticks() < target {
 		// Set the alarm to fire at the target tick count.