CXX C11 Atomic support

If you've got a C C11 library that's using `` and you want to use this library in CXX you simply can't. C++ provides a compatibility header `` but if you use the NuttX libcxxmini library it doesn't provide a atomic implementation. Unless you use of-course the std libraries but these tend to big.

It would interesting if we could have a minimal ``implementation for libcxxmini.

Do note it seems that C++23 partially fixes this incompatibility.
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0943r6.html

Would be interesting if it's possible to backport that.