Using compiler-rt as default rtlib needs libunwind.
However, currently libunwind is installed in /usr/$CTARGET/usr/lib and linker can't find it. 
Therefore, libunwind or llvm-libwind must be installed on target board by default during setup_board (same as gcc-libs).

x86_64-cros-linux-gnu-clang++ a.cc  -o a.out -rtlib=compiler-rt -fsanitize=address --sysroot=/build/amd64-generic 
/usr/x86_64-pc-linux-gnu/x86_64-cros-linux-gnu/binutils-bin/2.25.51/ld.gold.real: error: cannot find -lunwind
/usr/x86_64-pc-linux-gnu/x86_64-cros-linux-gnu/binutils-bin/2.25.51/ld.gold.real: error: cannot find -lunwind