Expand pyatomic coverage

[colesbury]

Feature or enhancement

Implementing PEP 703 requires use of atomic operations on more data types than provided by pycore_atomic.h. Additionally, pycore_atomic.h is only usable from Py_BUILD_CORE modules; it can't be used in public headers. PEP 703 will require atomic operations in object.h for Py_INCREF/DECREF, for example.

I propose adding a new header to wrap platform/C11 provided atomic operations:

• The functions will be exposed in the public C-API but prefixed with an underscore to indicate they are private. They need to be exposed because they will be used by other public inline functions.
• The functions operate on plain data types (i.e. int instead of _Py_atomic_int). This allows use of atomic operations on existing data types where we don't want to change the types of contained fields (i.e, the fields of PyTypeObject.) and avoids some issues regarding mixing C and C++.
• Use of GCC built-in atomics when available. These are available in GCC 4.8+. They do not require passing -std=gnu11 or -std=c11 to the compiler, which avoids breaking third-party extensions.
• The memory order is part of the function name instead of an argument. This means there are more function definitions (e.g., both _Py_atomic_load_int and _Py_atomic_load_int_relaxed) but keeps the implementation of each function simpler, particularly for MSVC. This reduces boiler-plate code when stepping through debug builds.

Linked PRs

• gh-108337: Add pyatomic.h header #108338
• gh-108337: Add pyatomic.h header #108701