Add basic mutex (lock) and spin lock implementations in C11, C++11, AVR, and STM32

[ElectricRCAircraftGuy]

Add basic mutex (lock) and spin lock implementations in C11, C++11, AVR, and STM32

Keep in mind that in AVR I may need to use atomic access guards (turn off interrupts, critical section, restore interrupts), and in both AVR and STM32 (any single-core system) I need to add safety anti-deadlock mechanisms such as automatic deferral, timeout, and retry.

References and research to get me started:

1. My answer on "Basic spin lock implementation in C11 using _Atomic types": https://stackoverflow.com/a/73819087/4561887
2. https://stackoverflow.com/questions/71626597/what-are-the-various-ways-to-disable-and-re-enable-interrupts-in-stm32-microcont/71626598#71626598
   1. Also the answer by Tom V about LDREX and STREX (exclusive load and store instructions): https://stackoverflow.com/a/71626777/4561887
   2. Example from mbedOS: https://github.com/ARMmbed/mbed-os/blob/master/rtos/source/Semaphore.cpp
3. https://en.wikipedia.org/wiki/Test-and-set#Pseudo-C_implementation_of_a_spin_lock
4. Google search for "STREX stm32"
5. YouTube: Spinlocks - Part 1 - A Basic Spinlock
6. http://blog.bachi.net/?p=11934
7. https://www.mikrocontroller.net/topic/358456 - mentions possible STM32 __LDREX() type C functions to mimic the assembly LDREX and STREX family of instructions which allow making mutexes
8. https://www.manualslib.com/manual/1375966/St-Stm32f4-Series.html?page=78
9. https://community.st.com/s/project/a8g3W000000fxUvQAI/strexldrex-is-not-an-intercore-synchronization-method
10. https://developer.arm.com/documentation/dui0553/b/?lang=en - download the "Cortex -M4 Devices Generic User Guide
    Generic User Guide" here!
11. https://en.cppreference.com/w/cpp/atomic/atomic
12. https://en.cppreference.com/w/c/language/atomic
13. https://en.cppreference.com/w/cpp/atomic/atomic/exchange
14. https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange
15. https://cplusplus.com/reference/atomic/atomic/compare_exchange_strong/
16. https://en.cppreference.com/w/c/language/atomic
17. https://en.cppreference.com/w/c/thread
18. Google search for "C atomic exchange"
19. Google search for __c11_atomic_exchange
20. https://www.gnu.org/software/libc/manual/html_mono/libc.html
21. Google search for "atomic_compare_exchange_weak"
22. https://en.cppreference.com/w/c/atomic/atomic_compare_exchange
23. https://en.cppreference.com/w/c/atomic/atomic_exchange
24. https://en.cppreference.com/w/c/atomic/atomic_compare_exchange
25. https://en.cppreference.com/w/c/thread
26. https://en.cppreference.com/w/c/atomic/ATOMIC_LOCK_FREE_consts - macros to see which atomic types in C are lock-free - THIS IS IMPORTANT TO ALLOW YOU TO assert in a library to ensure you're using truly lock-free types in any implementations you do for a spin lock!
27. https://en.cppreference.com/w/c/language/atomic
28. https://cplusplus.com/reference/atomic/atomic_exchange/
29. Google search for atomic spin lock in c
30. https://en.cppreference.com/w/cpp/atomic/atomic_flag - USE THIS in a library implementation I make for a spin lock
    22. https://en.cppreference.com/w/cpp/atomic/atomic_flag_test_and_set
31. https://en.cppreference.com/w/c/atomic/atomic_flag - no, use this one--this is the C documentation
    23. https://en.cppreference.com/w/c/atomic/atomic_flag_test_and_set
32. https://en.cppreference.com/w/c/thread#Atomic_operations

Important sections to look at in the [Cortex -M4 Devices Generic User Guide

Generic User Guide](https://developer.arm.com/documentation/dui0553/b/?lang=en):

p83-84:

p32-33: