arch/arm: add support for AArch64

AArch64 id the 64-bit variant for ARM.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Zhenqiang Chen <zhenqiang.chen@linaro.org>
Cc: Michael Hope <michael.hope@linaro.org>

 # Compute sparc-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="sparc64${CT_ARCH_SUFFIX}"

     fi

 }