[libstdc++, testsuite] Add dg-require-thread-fence

[Mike Stump]

On Oct 21, 2016, at 1:00 AM, Christophe Lyon <christophe.lyon@linaro.org> wrote:
> 
> So if we say that the current behaviour has to keep being the default,
> so that users think about what they are really doing, 

Having a toolchain not work by default to force users to think, isn't a winning strategy.

Everything should always, just work.  Those things that don't, we should fix.