[cxx-mem-model] sync_mem_exchange implementation with memory model parameters

[Andrew MacLeod <amacleod at redhat dot com>]

OK, I've brought the cxx-mem-model branch up to mainline as of this 
afternoon.  (After I applied the patch which renamed all the mis-named 
_sync expanders)

So this is the marginally reworked patch we had before which I think 
satisfies all the requests I've seen.

Bootstraps and no regerssions on x86_64-unknown-linux-gnu.

Assuming everyone is OK with this, I'll check it in and use it as a 
template for all the other __sync which need a memory model parameter 
specified at the end of http://gcc.gnu.org/wiki/Atomic/GCCMM/CodeGen

Then I'll move on to the libstdc++ changes.

Andrew

Attachment: exchange.patch
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