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

[Ville Voutilainen]

On 20 October 2016 at 19:34, Jonathan Wakely <jwakely@redhat.com> wrote:
> Either way, I don't think disabling 50% of the testsuite is the
> answer. If you don't like the failures, configure the library to build
> without threadsafe statics, or configure it to depend on libatomic, or
> something else. (We might want new --enable-xxx switches to simplify
> doing that).
>

I think having to add a dg-require to *every* run-time test we have,
even and especially to ones that have *nothing* to do with
any threading is an indication that this approach might not be a good
way to solve the problem.