Run gcc:tarball CI job for merge requests

Run gcc:tarball CI job for merge requests

Running gcc:tarball CI job for merge requests is consistent with how we
run gcc:out-of-tree CI job and should help identify problems with the
build system during the review process, not once merged during daily
runs. For the sake of time, unit and system tests associated with the
gcc:tarball CI job are excluded from merge requests.

Also, make sure that the tarball-create CI job includes the
default_triggering_rules anchor (as it is on the main branch), otherwise
adding the gcc:tarball CI job to merge request-triggered pipeline fails
with:

    Found errors in your .gitlab-ci.yml: 'gcc:tarball' job needs 'tarball-create' job but it was not added to the pipeline

(cherry picked from commit 83617cea9a7ac4b877b0874d041d21d80eb36fac)