_Atomic in C++

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When built as C++, `_Atomic` in mimalloc is defined as `std::atomic`. For Clang, `mi_process_attach()` in `prim.c` is declared so it runs before static ctors. The result is that any file level atomic values that happen be set during `mi_process_attach()` are then stomped by the std::atomic ctors, which zero init their values.
For example, in `page-map.c`:
```
static _Atomic(mi_bfield_t) mi_page_map_commit;
```

Start by reading prim.c around mi_process_attach() and page-map.c around the static mi_page_map_commit declaration, then trace how C++ std::atomic initialization interacts with the attach hook. Confirm the behavior with the existing build or reproduction path for Clang; done means values set during mi_process_attach() are not overwritten by later atomic construction.

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