We decided not to move with --target-arch yet when using bases as we want to have an improved experience.
Sorry for the disruption, but it should be possible to use this with --destructive-mode still. The reason for moving out of this is that there are many corner cases presented with cross compilation and the experience is not that great for general use.
Using both --target-arch=armhf and --destructive-mode for my snap on my 18.04.1 LTS host results in “Sorry, an error occurred in Snapcraft”. The trace.txt file shows that the make plugin throws a NotImplementedError (“does not support cross-compiling to a different architecture”). I think that snapcraft v2.x printed this warning directly to STDOUT, so this might be a slight DX improvement.