builddeb: Change architecture detection fallback to use dpkg-architecture

We currently use dpkg --print-architecture, which reports the
architecture of the build machine.  We can make a better guess
than this by asking dpkg-architecture what the host architecture,
i.e. the default architecture for building packages, is.  This is
sensitive to environment variables such as CC and DEB_HOST_ARCH,
which should already be set in a cross-build environment.

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