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 <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>

scripts/package/mkdebian

···
 			debarch=sh4$(if_enabled_echo CONFIG_CPU_BIG_ENDIAN eb)
 		fi
 		;;
+	um)
+		if is_enabled CONFIG_64BIT; then
+			debarch=amd64
+		else
+			debarch=i386
+		fi
+		;;
 	esac
 	if [ -z "$debarch" ]; then
 		debarch=$(dpkg-architecture -qDEB_HOST_ARCH)