[U-Boot] checkstack.pl

Masahiro,

Do you mind to add aarch64 support for checkstack.pl?

$ make checkstack /opt/linaro/gcc-linaro-aarch64-linux-gnu-4.8-2013.12_linux/bin/aarch64-linux-gnu-objdump -d u-boot $(find . -name u-boot-spl) | \ perl ./scripts/checkstack.pl aarch64 wrong or unknown architecture "aarch64"

York