Support C++23 atomic shims

[bc-lee]

In C++23, libc++'s <stdatomic.h> exposes std::atomic-based APIs instead of the C atomic interfaces that libdispatch's shim expects. That makes the existing _Atomic storage incompatible with atomic_load_explicit(), atomic_fetch_*(), and related calls when libdispatch is compiled as C++.

Route libdispatch's internal atomic operations through private _dispatch_atomic_* aliases. Clang C++23 keeps _Atomic storage and uses __c11_atomic_* builtins directly. GCC C++23 uses plain storage with __atomic_* builtins so the shim does not depend on public <stdatomic.h> names in that mode. C and older C++ modes continue to use <stdatomic.h>.

Fixes #940.

[compnerd]

ibdispatch is a C library, not a C++ library, so I'm not sure I understand why this should be needed.

[bc-lee]

libdispatch is primarily a C library, but this repo does compile C++ code, including src/block.cpp. That file includes internal.h -> shims.h -> shims/atomic.h, so a C++23 <stdatomic.h> incompatibility is relevant if that translation unit is built in C++23 mode.

[bc-lee]

The C23/C++23 atomic incompatibility is real. It was also mentioned in the Hacker News comment:

C _Atomic(T) and C++ std::atomic<T>. C++23 has C compatible header stdatomic.h that defines _Atomic(T), but it's still problematic

This repository exposes that problem in practice.

As I noted earlier, although libdispatch is primarily a C library, this repository does compile C++ code, including src/block.cpp. That means C++23 <stdatomic.h> behavior is relevant here.

I suggest that the maintainers reconsider this issue/PR.