toolchain-external: simplify previously-broken symbolic links

A previous commit rewrote broken symbolic links in staging, caused by a
non-singular ARCH_LIB_DIR. In this case, the symbolic links are typically
using one or more intermediate directory symlinks, which can be simplified
using the newly introduced simplify_symlink helper.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>