[cxx-mem-model] C++ wrappers

[Andrew MacLeod <amacleod at redhat dot com>]

Same as before, only a slight change.  __sync_mem_flag_test_and_set() 
and __sync_mem_flag_clear() have been removed in a different 
consolidation patch, so this changes those calls to 
__sync_mem_exchange() and __sync_mem_store() which are subsuming all the 
behaviour of those routines.

Straight up translation which now calls the new routines with a memory 
model parameter instead of the old __sync routines with various barriers.

bootstrapped on x86_64-unknown-linux-gnu and no new regressions.

OK for branch?

Attachment: c++.diff
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