external-toolchain: create lib64 symlinks if needed

Create lib64 -> lib and usr/lib64 -> usr/lib symbolic links in the
target and staging directories. This is needed for some 64 bits
toolchains such as the Crosstool-NG toolchains, for which the path to
the dynamic loader and other libraries is /lib64, but the libraries
are stored in /lib.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>