External toolchain support broken for devel/llvm38 but not devel/llvm37

It looks like there is something broken with the devel/llvm38 port or external toolchain support has regressed:


This works:
make  XCC=/usr/local/bin/clang37 XCXX=/usr/local/bin/clang++37 XCPP=/usr/local/bin/clang-cpp37  buildworld -j12 -s

This fails:
 make  XCC=/usr/local/bin/clang38 XCXX=/usr/local/bin/clang++38 XCPP=/usr/local/bin/clang-cpp38 buildworld -j12 -s

with:

/home/mmacy/devel/build/mnt/storage/mmacy/devel/drm-next-merge/tmp/usr/bin/ld: /usr/local/llvm38/bin/../lib/clang/3.8.1/lib/freebsd/libclang_rt.ubsan_standalone-x86_64.a: No such file: No such file or directory
clang-3.8: error: linker command failed with exit code 1 (use -v to see invocation)


mmacy_at_armageddon [~/devel/freebsd-ports-graphics/devel/llvm38|20:07|19] ls /usr/local/llvm38/lib/clang/3.8.1/        
include
mmacy_at_armageddon [~/devel/freebsd-ports-graphics/devel/llvm38|20:08|20] ls /usr/local/llvm37/lib/clang/3.7.1 
asan_blacklist.txt	include			lib