ci: Consistently use -j4 across x86 build jobs and -j8 on ARM.

ci: Consistently use -j4 across x86 build jobs and -j8 on ARM.

Our shared runners are set up for concurrent jobs ~= CPUs / 4 (x86) or 8
(ARM).  If you use more build processes than that, then jobs may be
fighting each other for shared system resources, possibly to the point of
failure (we've seen one of the runners OOM on some jobs before, though I'm
not sure if this was the cause).

To try to systematically prevent the problem, we make a ninja wrapper in
the containers that passes the -j flags, and set MAKEFLAGS in the
container builds.  This doesn't cover make in non-container builds, but I
believe we don't have any of those.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3782>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3782>