Issue #23644: Fix issues with C++ when compiling Python extensions

Disable completly pyatomic.h on C++, because <stdatomic.h> is not compatible with C++.

<pyatomic.h> is only needed by the optimized PyThreadState_GET() macro in
pystate.h. Instead, declare PyThreadState_GET() as an alias to
PyThreadState_Get(), as done for limited API.