Import libcxxrt b2396b5945d7a2697c4762c3e52dc6f732b2eebd from
https://github.com/pathscale/libcxxrt/.
Import revision c61efa043b14378efbd69c9a2686d44ed46ae179 of libcxxrt.
This adds __cxa_deleted_virtual, some explicit store barriers before
lock releases and fixes ARM guard variable initialisation.