gitlab: add a new build_unit job to track build size

gitlab: add a new build_unit job to track build size

We want to reduce the total number of build units in the system to get
on our way to a single binary. It will help to have some numbers so
lets add a job to gitlab to track our progress.

Cc: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-19-alex.bennee@linaro.org>