toolchain-external: support only one Linaro AArch64 toolchain at a time

Linaro toolchains are released so frequently (every month) that it
doesn't make much sense to support 3 consecutive versions. So, like we
do for ARM big-endian, let's support only one version at a time.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>