Fix PWM crashes on ESP8266

Vendor in the ESP8266 Arduino core PWM library, with a fix for a nasty
NMI crash bug.  Sometimes the NMI return instruction seems to fail,
resulting in an infinite loop as the PC gets stuck.  Work around this
by backing up and restoring the PC if needed.

wled00/wled.cpp

@@ -8,6 +8,8 @@
 #include "soc/rtc_cntl_reg.h"
 #endif
 
+extern "C" void usePWMFixedNMI();
+
 /*
  * Main WLED class implementation. Mostly initialization and connection logic
  */
@@ -408,6 +410,10 @@ void WLED::setup()
   DEBUG_PRINTF_P(PSTR("TX power: %d/%d\n"), WiFi.getTxPower(), txPower);
 #endif
 
+#ifdef ESP8266
+  usePWMFixedNMI(); // link the NMI fix
+#endif
+
 #if defined(WLED_DEBUG) && !defined(WLED_DEBUG_HOST)
   pinManager.allocatePin(hardwareTX, true, PinOwner::DebugOut); // TX (GPIO1 on ESP32) reserved for debug output
 #endif