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="YEM"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment="Sample i586 configuration tweaked to generate code for the geode."