dpkg-buildpackage: use build-arch and build-indep targets of debian/rules

'build-arch' is used when building only arch-any binaries (-B)
while 'build-indep' is used when building only arch-all binaries (-A).
To avoid breaking too many packages, dpkg-buildpackages verifies that
those targets are implemented by calling “make -f debian/rules -qn
<target>” and ensuring that it doesn't fail with exit code 2. Otherwise
it falls back to using the 'build' target.

This fallback is a temporary measure until all packages have been
converted to properly support the build-arch and build-indep targets.

Acked-by: Guillem Jover <guillem@debian.org>