Fix c.inc _Atomic define for C++

[mrdomino]

No description provided.

[mrdomino]

PR description (can be drag-select copied and pasted in github's UI)

c.inc (AFAICT erroneously) defined _Atomic(t) as `volatile t *`, when it should have just said `volatile t`, when __STDC_VERSION__ was too small. This happens when we’re compiling C++, but in C++11, _Atomic is a define supplied by the STL rather than a keyword supplied by the compiler. Wait though, it gets better: in C++11, _Atomic hooks you into the morass that is stdatomic.h, and ultimately refers everything back to std::atomic<T>.

The gory, horrifying details are in libcxx’s __atomic/cxx_atomic_impl.h. The tldr is that for our purposes it’s fine to just say volatile and use the normal libc/intrin/atomic.h functions.

[mrdomino]

#1229 compiles with this change, and not without it.

[mrdomino]

From cppreference on std::atomic:

Implementations are recommended to ensure that the representation of _Atomic(T) in C is same as that of std::atomic<T> in C++ for every possible type T. The mechanisms used to ensure atomicity and memory ordering should be compatible.

This gives us hope that it may be legitimate to go down this road…