[cxx-mem-model] options for controlling data races

[Aldy Hernandez]

The following patch adds the --param(eters) needed to tweak the 
optimizers into allowing or disallowing data races in cross-thread 
visible data.

There will be additional flags added later to make these parameters 
available to the common user.  It has been suggested to use 
-fmemory-model=xxx, but I will implement this later.

The next step will be to find out where we introduce the data races in 
the testsuite, and fix the optimizers accordingly.

Committed to branch.
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