[PATCH] Linux: Implement membarrier function

[Florian Weimer]

* Joseph Myers:

> Are you going to open a bug report in Bugzilla for the lack of 
> documentation (discussing exactly what the issues are that need to be 
> clarified to document the function)?

A filed a bug for the general issue of lack of documentation for the
memory model:

  <https://sourceware.org/bugzilla/show_bug.cgi?id=23955>

Thanks,
Florian