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

首先跟踪 get_gc_stats 以及存储 ts_start 和 ts_stop 的位置，然后检查 memcpy 路径，了解外部进程如何观察 GC 数据。先阅读链接的 PR gh-155828 以及任何相关测试；当外部采样在弱内存顺序平台上不再返回跨 GC 周期撕裂的快照时，即表示完成。