backporting PR52558 to 4.7?

[Aldy Hernandez]

Hello gentlemen.

Would it be ok to backport the fix for PR52558 into the 4.7 branch? 
This PR is the store data race patch I have been iterating with Richi. 
Doing so will avoid critical data races for both TM and the C++ memory 
model.

The code is all predicated by flag_tm or !PARAM_VALUE 
(PARAM_ALLOW_STORE_DATA_RACES)), so it shouldn't affect code paths 
outside of TM or the C++1x memory model.

Thanks.
Aldy