toolchain-external: add Arm AArch64 toolchain 8.2-2018.08

toolchain-external: add Arm AArch64 toolchain 8.2-2018.08

This is the same toolchain that was previously distributed by Linaro. [1]

Switch default toolchain as this toolchain supersed the Linaro AArch64 toolchain.
Only x86_64 host are supported, so keep Linaro toolchain for x86 host.

Tested with qemu_aarch64_virt_defconfig.

[1] https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>