[Patch, libstdc++] Fix data races in basic_string implementation

[Marc Glisse]

On Tue, 1 Sep 2015, Dmitry Vyukov wrote:

> The refcounted basic_string implementation contains several data races
> on _M_refcount:

There are several bug reports about races in basic_string in bugzilla 
(some might even have been closed as wontfix because of the new 
implementation). Does this also fix some of them?

(ChangeLog entry appears to be missing)

-- 
Marc Glisse