fix(install_toolchain): Prevent tar from replacing lib symlinks

The cross toolchain doesn't have lib symlinks but this works because it
also doesn't install things to lib. However when this script manually
extracts the toolchain packages to BOARD_ROOT tar includes those empty
directories, replacing the symlinks that were previously there.

Longer term this really needs to go away, whatever build time is saved
by re-using the cross toolchain packages in BOARD_ROOT is not worth this
insane level of complexity it causes...