toolchain-external: add missing symlink for Linaro toolchains

In 11ec38b6 ("toolchain-external: fix Linaro ARM toolchain
support"), we fixed the support for Linaro EABIhf toolchains by adding
a /lib/arm-linux-gnueabihf -> /lib symbolic link. This is needed
because the dynamic loader looks for libraries in
/lib/arm-linux-gnueabihf rather than the usual /lib, but Buildroot
installs all libraries in /lib.

However, we forgot that the dynamic loader also loads libraries from
/usr/lib/arm-linux-gnueabihf rather than /usr/lib, so this patch fixes
that by adding the necessary symbolic link.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>