gitlab: use template for ubuntu-24.04-aarch64 jobs

gitlab: use template for ubuntu-24.04-aarch64 jobs

Most of the test is pure boilerplate so to save ourselves from
repetition move all the main bits into a minimal copy of
native_build_job_template but without the caching.

We keep all the current allow_fail and configure setups but do take
the opportunity to replace the -j`nproc --ignore=40` hack with
something that almost, but not quite, saturates the machine its being
built on.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-ID: <20251016150357.876415-2-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>