Rename arch-aarch64 to arch-arm64

Build instruction:
  ./build/tools/download-toolchain-sources.sh /tmp/ndk-$USER/src
  ./build/tools/gen-platforms.sh --minimal --dst-dir=$NDK \
     --ndk-dir=$NDK --overlay --arch=arm64
  ./build/tools/build-gcc.sh /tmp/ndk-$USER/src $NDK \
     aarch64-linux-android-4.8 --try-64

Change-Id: Ieaac9908cd2121981e257113cb8d5d3575c8718e
22 files changed