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 Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_SUFFIX:-${CT_ARCH_ALPHA_VARIANT}}"

}