arch: use $(obj)/ instead of $(src)/ for preprocessed linker scripts

These are generated files. Prefix them with $(obj)/ instead of $(src)/.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>