add simplistic stdatomic.h compatible with MSVC

Only the functions needed with the current code are defined.

The header will not work with other compilers. It will need to be updated for each compiler that doesn't have stdatomic.h.

I checked the of MSVC and it doesn't really handle the memory order. Only for ARM it does things a little bit different. Also it doesn't use the InterlockedXXX API but internal things we're not suppose to use directly.