Torn reads of GC statistics from external process via get_gc_stats

Bug report

Bug description:

If an external tool sampling GC statistics does not pause the target process (and we designed get_gc_stats not to pause), it can read torn data under certain circumstances.

There are two distinct problems:

1. Weakly ordered platforms: The stores of ts_start and ts_stop may be reordered by the CPU/optimizer. We need to add memory barriers to mitigate this.

2. Large memcpy window: We are copying a sufficiently large memory region via memcpy. If two or more GC cycles occur during this copy, we can end up with inconsistent data spanning multiple GC states.

@maurycy has an idea to use a sequence counter for consistency checks — I think he should definitely give it a try.

cc @pablogsal

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155828

Empieza rastreando get_gc_stats y las ubicaciones que almacenan ts_start y ts_stop; después, revisa la ruta de memcpy para ver cómo un proceso externo observa los datos de GC. Lee primero el PR vinculado gh-155828 y cualquier prueba relacionada; se considera terminado cuando el muestreo externo deja de devolver snapshots desgarrados entre ciclos de GC en plataformas con orden de memoria débil.