ARM: global_data: make set_gd() work for armv5 and armv6

The Thumb instruction `ldr` is able to move high registers only from
armv7. For armv5 and armv6 we have to use `mov`.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>