Add support for 64-bit TAs

common: Add CROSS_COMPILE_ta_arm## flags
hikey and mediatek: Update with new TA dev kit name

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>