re2: refactor atomic operations interface + use compiler builtin atomics for newer compilers The purpose of this change is to make atomic operations interceptable by race detectors. Tested with gcc 4.6.3, gcc 4.9.0 (20130603) and clang 3.4 (184078), in both release and debug builds. R=rsc CC=kcc, re2-dev https://codereview.appspot.com/9586043 
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