Implement a CI system

This sets up autobuild with the images from https://gitlab.dune-project.org/joe/dune-perftool-ci.

Two workarounds were required:

• The opcounter sometimes triggered a clang bug, see #89 (closed). This is worked around in 13e2bfa1.
• The TSC expects to fish the TSC frequency out of the kernel messages using dmesg on Linux. This can fail for many reasons, i.e. the relevant messages have dropped out of the kernel message buffer, or the kernel does not grant access to the messages for non-root. We work around this for the CI by replaing dmesg with a script that always outputs a TSC frequency of 1GHz, see joe/dune-perftool-ci@b4d471b7.

Closes: #89 (closed).