Few updates, inc. stackclash warning fix

Hi Andres, here are the three pending changes I've got. This MR is mainly due to the first one, since it was a bit more disruptive than we anticipated.

First commit is the stackclash-warning fix for armhf. I had to move things down to below include .../architecture.mk because we need DEB_HOST_ARCH to be set.

(I also tweaked the description on optimize=-lto, since the real problem is that Ubuntu's default LTO flags give you full/fat LTO, and with Chromium that makes linking stupidly expensive)

The second commit has the cross-build improvements suggested by Helmut Grohne.

The third commit is what we talked about 😉