Since contrib/libcxxrt's ancestry was never correct, subversion 1.8 and

Since contrib/libcxxrt's ancestry was never correct, subversion 1.8 and
higher cannot merge to it from the vendor area.

Re-bootstrap the ancestry by doing (apologies to your retinas):
* svn pg svn:mergeinfo contrib/libcxxrt > mergeinfo.txt
* svn rm contrib/libcxxrt
* svn cp ^/vendor/libcxxrt/dist@297299 contrib/libcxxrt
* svn rm CMakeLists.txt doxygen_config
* svn cp ^/head/contrib/libcxxrt/FREEBSD-upgrade@297299 contrib/libcxxrt
* svn cp ^/head/contrib/libcxxrt/exception.cc@297299 contrib/libcxxrt
* svn cp ^/head/contrib/libcxxrt/unwind-arm.h@297299 contrib/libcxxrt
* svn pd -R svn:mergeinfo contrib/libcxxrt
* svn ps -F mergeinfo.txt svn:mergeinfo contrib/libcxxrt