Merge 47661d00cd4d6cd728ae31b0bb29a49a6c06272a

Force consistent alignemnt of _Unwind_Exception with libgcc_s.

Import new snapshot of libcxxrt. Fixes demangling of anonymous
namespaces. Avoid use of old style (C) casts.

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