crosstool-ng: fixup after copy_toolchain_lib_root api changes

Commit 0729b544 (Improve external toolchain logic to support IA32
Sourcery CodeBench toolchain) and e1f0804c (external-toolchain: add
support for recent Linaro toolchains) changed the interface of
copy_toolchain_lib_root, but ctng wasn't updated so libraries weren't
copied to the target.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>