Move all code paths touched by NMI to IRAM

Prerequisite for calling Cache_Read_Disable() for OTA updates.

Massive thanks due to @foogod and their xtobjdis tool, creating the call
graph of where the wDev_ProcessFiq touched (including in newlib, etc.)
would have otherwise been very painful:
https://bitbucket.org/foogod/xtobjdis

FreeRTOS/Source/portable/esp8266/port.c

@@ -265,7 +265,7 @@ void IRAM vPortExitCritical( void )
 
 _xt_isr isr[16];
 
-void _xt_isr_attach(uint8_t i, _xt_isr func)
+void IRAM _xt_isr_attach(uint8_t i, _xt_isr func)
 {
     isr[i] = func;
 }