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

[Mike Stump]

On Oct 20, 2016, at 9:51 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> If Every. Single. Test. that uses the libstdc++ library has this
> failure, and the library can't be made to be usable, the answer is
> surely to change the meaning of "dg-do run" to not link+run tests, not
> add a new directive to Every. Single. Test.  (and every single test we
> add in future too!)

So, from a test suite perspective, the correct fix it to make the port just work.  I have a bare metal port, I test libstdc++.
I'd be curious to hear from the arm folks about it.