ci: add s390x vmtest

Run it on the self-hosted builder with tag "z15".

Also add the infrastructure code for the self-hosted builder.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/vmtest/s390x-self-hosted-builder/README.md

@@ -0,0 +1,72 @@
+# IBM Z self-hosted builder
+
+libbpf CI uses an IBM-provided z15 self-hosted builder. There are no IBM Z
+builds of GitHub Actions runner, and stable qemu-user has problems with .NET
+apps, so the builder runs the x86_64 runner version with qemu-user built from
+the master branch.
+
+## Configuring the builder.
+
+### Install prerequisites.
+
+```
+$ sudo dnf install docker
+```
+
+### Add services.
+
+```
+$ sudo cp *.service /etc/systemd/system/
+$ sudo systemctl daemon-reload
+```
+
+### Create a config file.
+
+```
+$ sudo tee /etc/actions-runner-libbpf
+repo=<owner>/<name>
+access_token=<ghp_***>
+```
+
+Access token should have the repo scope, consult
+https://docs.github.com/en/rest/reference/actions#create-a-registration-token-for-a-repository
+for details.
+
+### Autostart the x86_64 emulation support.
+
+```
+$ sudo systemctl enable --now qemu-user-static
+```
+
+### Autostart the runner.
+
+```
+$ sudo systemctl enable --now actions-runner-libbpf
+```
+
+## Rebuilding the image
+
+In order to update the `iiilinuxibmcom/actions-runner-libbpf` image, e.g. to
+get the latest OS security fixes, use the following commands:
+
+```
+$ sudo docker build \
+      --pull \
+      -f actions-runner-libbpf.Dockerfile \
+      -t iiilinuxibmcom/actions-runner-libbpf
+$ sudo systemctl restart actions-runner-libbpf
+```
+
+## Removing persistent data
+
+The `actions-runner-libbpf` service stores various temporary data, such as
+runner  registration information, work directories and logs, in the
+`actions-runner-libbpf` volume. In order to remove it and start from scratch,
+e.g. when switching the runner to a different repository, use the following
+commands:
+
+```
+$ sudo systemctl stop actions-runner-libbpf
+$ sudo docker rm -f actions-runner-libbpf
+$ sudo docker volume rm actions-runner-libbpf
+```