Import libcxxrt 47661d00cd4d6cd728ae31b0bb29a49a6c06272a

The repository moved to https://github.com/libcxxrt/libcxxrt in the mean
time, but keep it on the same branch as before. This primarily bring

Import libcxxrt 47661d00cd4d6cd728ae31b0bb29a49a6c06272a

The repository moved to https://github.com/libcxxrt/libcxxrt in the mean
time, but keep it on the same branch as before. This primarily brings
C++14 support.

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Import revision c61efa043b14378efbd69c9a2686d44ed46ae179 of libcxxrt.
This adds __cxa_deleted_virtual, some explicit store barriers before
lock releases and fixes ARM guard variable initialisation.

Import libcxxrt b2396b5945d7a2697c4762c3e52dc6f732b2eebd from
https://github.com/pathscale/libcxxrt/.