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>

scripts/package/mkdebian

@@ -42,13 +42,13 @@ set_debarch() {
 		fi
 		;;
 	*)
-		debarch=$(dpkg --print-architecture)
+		debarch=$(dpkg-architecture -qDEB_HOST_ARCH)
 		echo "" >&2
 		echo "** ** **  WARNING  ** ** **" >&2
 		echo "" >&2
 		echo "Your architecture doesn't have its equivalent" >&2
 		echo "Debian userspace architecture defined!" >&2
-		echo "Falling back to using your current userspace instead!" >&2
+		echo "Falling back to the current host architecture ($debarch)." >&2
 		echo "Please add support for $UTS_MACHINE to ${0} ..." >&2
 		echo "" >&2
 		;;