More Nim on more Microcontrollers!? (Arm CMSIS / Zephyr RTOS)

[shirleyquirk]

The `_IOM` etc expand to `volatile` or `volatile const` This can either be done 
with `codegendecl` or with `volatileLoad` and `volatileStore` from 
[volatile](https://nim-lang.org/docs/volatile.html)