GitLab CI: use Ninja on MacOSX

remove gold linker everywhere

Like with ccache, we want to make sure the git checkout is considered
clean during build.

Placing these directories at the root will make the git revision
detection consider the working directory to be dirty.

Ring Racers' object file output is considerably larger than SRB2.
50M is insufficient to hold even a single build.

The singular .gitlab-ci.yml is pretty beefy. Splitting it into
smaller files by job and job template will make maintaining the
individual jobs easier.