Potentially merging cxx-mem-model with mainline.

[Aldy Hernandez]

> It does not address other missing aspects of the c++ memory model.  In
> particular, bitfields are still not compliant with not introducing new
> potential data races.

Can we treat this as a bugfix to be done during stage2?  There is
already some support in mainline, but it performs lousy on anything but
the most simple of testccases.  After much iterations with Richi, we
couldn't come to an agreement on the remaining fixes.  I would like to
take another stab at this during stage2, if it is OK with Richi and
others.