Import EBR mechanism from https://github.com/rmind/libqsbr

* Make implementation header only
* Opaque implementation in namespace
* Use std::atomic instead of custom implementation
* Modify ringbuf test to work with c++ changes

Imported from https://github.com/llvm-mirror/libcxx/commit/88f5d7a38bf379d2772a7c0b064b680d6eaea4dc