Use of busy-wait inside IRAM_ATTR function: core_esp8266_waveform_pwm.cpp

@earlephilhower Please check the busy-wait (idle == false)
```
while (pwmState.pwmUpdate) {
if (idle) {
delay(0);
}
MEMBARRIER();
}
```
from `static IRAM_ATTR void _notifyPWM(PWMState *p, bool idle)`
from `IRAM_ATTR bool _stopPWM_weak(uint8_t pin)`

A. This causes a busy wait inside an ISR (#8043).
B. The IRQ for PWM (NMI) does interrupt other ISRs that may be busy-waiting in notifyPWM. OK.
C. Won't any other GPIO interrupt still be blocked by the waiting ISR in A? Therefore, depending on how long PWM needs to clear the ISR in A to continue in `_notifyPWM`, every GPIO IRQ gets delayed? Does this leave room for acceptable responsiveness?