kbuild: deb-pkg: add debarch for ARCH=um

'make ARCH=um bindeb-pkg' shows the following warning.

  $ make ARCH=um bindeb-pkg
     [snip]
    GEN     debian

  ** ** **  WARNING  ** ** **

  Your architecture doesn't have its equivalent
  Debian userspace architecture defined!
  Falling back to the current host architecture (amd64).
  Please add support for um to ./scripts/package/mkdebian ...

This commit hard-codes i386/amd64 because UML is only supported for x86.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>