Properly support <atomic> on AVR and Cortex-M

[salkinium]

• [x] Implement all atomics library calls for AVR and ARM Cortex-M
• [x] https://github.com/modm-io/avr-libstdcpp/pull/34
• [x] Add unit test for \<atomic>
• [x] Hardware testing
  • [x] STM32H7
  • [x] STM32G0
  • [x] ATmega2560
• [x] Inline most of the atomic library calls for significant overhead reduction
• [x] Correct memory order support for single- and multi-core devices
• [x] Fix the static initialization guard (again)
• [x] Documentation