Fix PWM crashes on ESP8266

[willmmiles]

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 "returning" to the NMI handler itself. Work around this by backing up and restoring the PC (and PS) from the last known good target values if needed.

I've opted to place the code in the platformio "libs" folder, which makes it easy to conditionally include for ESP8266 builds via platformio.ini.

Fixes PWM crash issues issues described in #3962.

[blazoncek]

@tablatronix @softhack007 @gaaat98 can you test this PR please?

[gaaat98]

Hi! Sorry for the late reply but I was travelling. I did not do any stress test, but I can say that with the fix, the ESP8266 PWM light did not crash when UDP synced with an ESP32, even with the clock speed set to fastest. Before, it crashed in a few seconds, but now I was able to have it working for more than 10 minutes with sound reactive effects going! Thanks a lot for your great work and research effort!!

[blazoncek]

@softhack007 any second thoughts before merging?
@tablatronix were you able to do any testing?

[softhack007]

👍 looks good for me, thanks

[tablatronix]

I have not, my rgb lights are now in my pool, but I just got some small ones for bench testing. Will try to try to wire a tester up

[Moonbase59]

FYI: This apparently also fixed my Athom 15W bulbs crashing every few seconds when controlled by LedFx. Installed WLED_0.15.0-b5_ESP02_160.bin.gz on these, and crashing problems are gone. Thanks!