rust: sync: add `Arc::ptr_eq`

Add a method for comparing whether two `Arc` pointers reference the same
underlying object.

This comparison can already be done by getting a reference to the inner
values and comparing whether the references have the same address.
However, writing `Arc::ptr_eq(a, b)` is generally less error-prone than
doing the same check on the references, since you might otherwise
accidentally compare the two `&Arc<T>` references instead, which wont
work because those are pointers to pointers to the inner value, when you
just want to compare the pointers to the inner value.

Also, this method might optimize better because getting a reference to
the inner value involves offsetting the pointer, which this method does
not need to do.

Co-developed-by: Wedson Almeida Filho <walmeida@microsoft.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230517200814.3157916-1-aliceryhl@google.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>