toolchain-external: add Linaro ARM big endian toolchain

Linaro has started to release ARM big endian toolchains, so we
integrate this toolchain in the external toolchain logic of
Buildroot. Since ARM big endian is probably going to be a lot more
uncommon than ARM little endian, we will only support one version at a
time of this toolchain.

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