Fix data race in Arc and Weak types

[Ollie-Pearce]

Refcount decrements for Arc use Relaxed. This can result in a data race when two threads drop an owned instance of a type duplicated via Clone.

Reproduction:

Test Case:

#[test]
fn racy_test() {
    let a: Arc<u64, RustAlloc> = Arc::new_in(7, RustAlloc::new());
    let b = a.clone();
    std::thread::scope(|s| {
        s.spawn(move || drop(b));
        drop(a);
    });
}

Running with Miri:

MIRIFLAGS="-Zmiri-many-seeds=0..16" cargo +nightly-2025-08-20 miri test -p stabby-abi --test repro

Miri output:

error: Undefined Behavior: Data race detected between (1) atomic store on thread `race_arc_drop_v` and (2) deallocation on thread `unnamed-3` at alloc688906+0x20
    |
 82 | /         alloc_rs::alloc::dealloc(
 83 | |             dealloc_start,
 84 | |             core::alloc::Layout::from_size_align_unchecked(prev_layout.size, prev_layout.align),
 85 | |         )
    | |_________^ (2) just happened here

Fix:

Synchronise drops of std::sync::Arc using a release ordering to decrement the reference counter and a fence before memory is freed. This mirrors the synchronisation used by std::sync::Arc