armv8: update gd after relocate

We need to update gd in assamble code after relocate,
this is a fix to:
adc421e4 arm: move gd handling outside of C code

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>