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

[Mike Stump]

On Oct 20, 2016, at 5:20 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> 
> I am considering leaving this in the ARM backend to force people to
> think what they want to do about thread safety with statics and C++
> on bare-metal systems.

Not quite in the GNU spirit?  The port people should decide the best way to get as much functionality as possible and everything should just work, no sharp edges.

Forcing people to think sounds like a sharp edge?