Auto merge of #95183 - ibraheemdev:arc-count-acquire, r=Amanieu

Weaken needlessly restrictive orderings on `Arc::*_count`

There is no apparent reason for these to be `SeqCst`. For reference, [the Boost C++ implementation relies on acquire semantics](https://github.com/boostorg/smart_ptr/blob/f2cc84a23c64b8a73c9b72b34799d0854d7e0787/include/boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp#L137-L140).

diff --cc library/alloc/src/sync.rs
Simple merge