Need to keep additional build artifacts

[Michi Henning]

Opening a bug for this on request from robru.

We are about to switch to abi-compliance-checker for ABI testing. It generates a nice HTML report with all the details. If, during a build on Jenkins CI or in a silo, we have an ABI break on, say, PowerPC only, we get a test failure, but we don't know the details without seeing the generated report.

Proposal: we dump the reports into a directory "reports" in the source tree. (We can do that prior to the build step.) The CI infrastructure could then pick up whatever is in that directory and save it, so we can download the reports after a build to figure out what's wrong.