impatomic: add strong/weak compare_exchange

impatomic: add strong/weak compare_exchange

According to n2748.html, there was a change to allow spurious failure of
compare_exchange operations. Thus, there is no longer a
compare_exchange() operation, but rather a pair of
compare_exchange_{strong,weak)(). I resolve this for compilation
purposes now by editing the impatomic header (mostly with Vim macros for
patterned copy-paste-substitute) and adding an _ATOMIC_CMPSWP_WEAK_
macro, which is just an alias for _ATOMIC_CMPSWP_. Thus, we don't
simulate spurious failure yet.

See:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2748.html