Add option to use spin instead of spinning_top

I know this is backtracking on the PR from 2020, but it seems that `spin` is once again maintained, moreso now than `spinning_top`.

My project uses spin, as does some of its dependencies (e.g. `google/percore`). It would be nice if I didn't need to introduce a new Spinlock type into my project just to use linked-list-allocator.