Add ICACHE_RAM_ATTR compatibility macro with esp8266

[copercini]

This help the ESP8266 libraries to run on ESP32

[copercini]

or maybe put it here: https://github.com/espressif/arduino-esp32/blob/master/tools/sdk/include/esp32/esp_attr.h

[me-no-dev]

let's add a file called "esp8266-compat.h" and put that and ICACHE_FLASH_ATTR and maybe more in near future :) include that header into Arduino.h

[copercini]

Nice idea!

The strangest thing is that there are some ICACHE_FLASH_ATTR calls in the actual ESP32 core:
https://github.com/espressif/arduino-esp32/search?q=ICACHE_FLASH_ATTR

[me-no-dev]

it evaluates to nothing and prolly not called anywhere to get the undefined error :)