efi_loader: Handle memory overflows

jetson-tk1 has 2 GB of RAM at 0x80000000, causing gd->ram_top to be zero.
Handle this by either avoiding ram_top or by using the same type as
ram_top to reverse the overflow effect.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Alexander Graf <agraf@suse.de>