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>

reporter_name="Alexander BIGGA"

reporter_url="http://sourceware.org/ml/crossgcc/2008-06/msg00031.html"

reporter_comment=""