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

[Mike Stump]

On Nov 14, 2016, at 12:31 PM, Ramana Radhakrishnan <ramana.gcc@googlemail.com> wrote:
> 
> https://sourceware.org/ml/newlib/2015/msg00653.html

I think that patch is wrong.  It is wrong to warn on a system where an empty body is correct.  It is wrong to warn when something more than nothing needs to be done.  In short, it is never right.

If you implement what is required for any machine (configuration), at least it will be right for that configuration.  Others where that isn't correct, can then implement what is correct for their machine (configuration), if you cannot.