Fix Debian packaging on ARMv7/ARM64

Fix Debian packaging on ARMv7/ARM64

When building packages on Debian-based systems specify the target
architecture used by 'alien' to convert .rpm packages into .deb: this
avoids detecting an incorrect value which results in the following
errors:

<package>.aarch64.rpm is for architecture aarch64 ; the package cannot be built on this system
<package>.armv7l.rpm is for architecture armel ; the package cannot be built on this system

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes zfsonlinux/zfs#7046
Closes #678