chore: use stabilized AtomicXXX::from_ptr() (#1738)

# What does this PR do?

This PR replaces a raw cast to `AtomicUsize` to a call to `Atomic::from_ptr`, which is the official solution for this use-case (have an atomic view of a raw pointer)

# Motivation

Came to this line looking at something else. The comment mentions `from_ptr()` is unstable, which isn't anymore (since Rust 1.75). So let's pay the technical debt (one nickel at a time!).

# Additional Notes

N/A

# How to test the change?

N/A

Co-authored-by: yann.hamdaoui <yann.hamdaoui@datadoghq.com>

Author: yannham

datadog-ipc/src/platform/unix/mem_handle_macos.rs

@@ -163,8 +163,7 @@ impl<T: FileBackedHandle + From<MappedMem<T>>> MappedMem<T> {
                 (MAPPING_MAX_SIZE - usize::from(page_size)) as off_t,
             )
             .unwrap();
-            // AtomicUsize::from_ptr() is still unstable
-            let size = ptr.cast::<AtomicUsize>().as_ref();
+            let size = AtomicUsize::from_ptr(ptr.cast::<usize>().as_ptr());
             size.fetch_max(handle.get_size(), Ordering::SeqCst);
             _ = munmap(ptr, usize::from(page_size));
         }