[platform] add arm64 toolchain file for cmake (#1000)
[platform] add arm64 toolchain file for cmake (#1000)

Added the arm64 toolchain file. This enabled the cross compile based on NXP
i.MX8M Linux SDK.

TEST:  source ${PATH_TO_NXP_LINUX_SDK}/environment-setup-cortexa53-crypto-poky-linux
       ./script/cmake-build -DOTBR_BORDER_ROUTING=ON -DOTBR_WEB=ON \
        -DBUILD_TESTING=OFF -DOTBR_DBUS=ON -DOTBR_DNSSD_DISCOVERY_PROXY=ON \
        -DOTBR_SRP_ADVERTISING_PROXY=ON -DOT_THREAD_VERSION=1.2 \
        -DOTBR_INFRA_IF_NAME=mlan0 \
        -DCMAKE_TOOLCHAIN_FILE=./examples/platforms/nxp/linux-imx/aarch64.cmake
openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix