Use "uname -m" instead of "arch"

According to "arch"'s manpage, it's identical to "uname -m".

Some distros ship uname but don't ship arch, however all distros ship uname,
therefore it makes sense to use "uname -m" whenever possible.

Signed-off-by: Christian Bühler <christian@cbuehler.de>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>