toolchain-external: Linaro AArch64 toolchains need a symlink, like ARM ones

Like ARM ones, the Linaro AArch64 toolchains expect libraries in
/lib/aarch64-linux-gnu and /usr/lib/aarch64-linux-gnu, but Buildroot
always installs them in /lib and /usr/lib. Therefore, this commit adds
the appropriate symbolic links, just like we're already doing for
Linaro ARM toolchains.

Reported-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>