added package_sparkfun_index.json entry for aarch64 Support

[rmcsqrd]

In reference to Apollo3 Core Issue #105:

I was having issues getting my arduino code to compile on my Jetson Nano. Adding

            {
              "host": "aarch64-linux-gnu",
              "url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2018-q2-update-linuxarm64.tar.bz2",
              "archiveFileName": "gcc-arm-none-eabi-7-2018-q2-update-linuxarm64.tar.bz2",
              "checksum": "SHA-256:6fb5752fb4d11012bd0a1ceb93a19d0641ff7cf29d289b3e6b86b99768e66f76",
              "size": "99558726"
            },

to the package index .json file allowed my code to compile.