gitlab: add build-user-hexagon test

We special case this as the container with the cross compiler for the
tests takes so long to build it is manually uploaded into the
registry.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210512102051.12134-9-alex.bennee@linaro.org>