Add atomic builtins implementation and example

[salkinium]

I'm adding the cleaned up modm implementation of the atomics based on a critical section.

• [x] Complete atomic builtins implementation
• [ ] More interesting example
• [ ] Fix documentation
• [ ] Fix issues with compilation on avr-gcc 12 regarding --param=min-pagesize=0

I just threw this together, I need to polish this some more, probably next weekend. cc @ckormanyos