RPI3 can't build kernel-toolchain

[bob prohaska]

On a Pi3 running r328935 trying to build sources at 329171 a 
make -j2 kernel-toolchain fails with
/usr/bin/ld: error: duplicate symbol: 
>>> defined at ASTImporter.o:() in archive /usr/obj/usr/src/arm64.aarch64/tmp/obj-tools/lib/clang/libclang/libclang.a

Tried updating the source tree and didn't seen any relevant changes,
no luck using the -DNO_CLEAN option. The machine can't seem to get
very far with j > 2, so cleaning will slow things by days.

I'll try cleaning by degrees, but am unclear on the order. Is the sequence
from least-to-most-thorough something like:

make clean
make cleandir
2nd make cleandir
rm -rf /usr/obj/usr/

The make logfile is at
http://www.zefox.net/~fbsd/rpi3/crashes/20180212/toolchain.log

Thanks for reading, and any guidance!

bob prohaska