builddeb: Skip architecture detection when KBUILD_DEBARCH is set

If KBUILD_DEBARCH is set then we will not use the result of
architecture detection, and we may also warn unnecessarily.
Move the check for KBUILD_DEBARCH further up to avoid this.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>