Prepare toolchain for ARM64

Download toolchain for ARM64:

gcc-linaro-6.4.1-2017.11-x86_64_aarch64-linux-gnu.tar.xz

Install toolchain into /opt

tar xvf gcc-linaro-6.2.1-2016.11-x86_64_aarch64-linux-gnu.tar.xz -C /opt/

Export environment variables for compiling

export PATH=$PATH:/opt/gcc-linaro-6.4.1-2017.11-x86_64_aarch64-linux-gnu/bin
export ARCH=arm64
export CROSS_COMPILE=/opt/gcc-linaro-6.4.1-2017.11-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-