Tweaking atomics and memory barriers in shared_ptr

[Paolo Carlini]

Hi Jonathan,
> thanks for the review.  I've committed the patch without the new
> _S_single specialisations.  They could always be added later if there
> is any demand for a non-threadsafe shared_ptr, and if there isn't
> demand then you're right not to change the code.
>   
Totally agreed, thanks again for now.
> With the new memory barrier usage and the comments I added, are you
> happy if I close http://gcc.gnu.org/PR24347 ?
>   
Sure. At some point it would be nice to also add something to the docs
proper. I think Benjamin is already at work on a documentation overhaul,
you may want to get in touch with him...
> Now that this patch is done I'll prepare the patch for aliasing and
> allocator support.
>   
Great!
Paolo.