[libstdc++] remove __compare_and_swap

[Mark Mitchell]

> Okay.  Given the confusion that __c_a_s has caused for port maintainers,
> I suggest leaving it out until we actually try introducing MI support into
> linked list structures.  It would be a pain to require new ports to submit
> a function that may or may not need rewriting at some arbitrary future
> date.

I agree.

-- 
Mark Mitchell                mark@codesourcery.com
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