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>

.gitlab-ci/container/container_pre_build.sh

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+# Common setup among container builds before we get to building code.
+
 export CCACHE_COMPILERCHECK=content
 export CCACHE_COMPRESS=true
 export CCACHE_DIR=/cache/mesa/ccache
@@ -11,3 +13,17 @@ export CC="/usr/lib/ccache/gcc"
 export CXX="/usr/lib/ccache/g++"
 
 ccache --show-stats
+
+if uname -m | grep -q arm || uname -m | grep -q aarch64; then
+    export JFLAGS=-j8
+else
+    export JFLAGS=-j4
+fi
+
+# Make a wrapper script for ninja to always include the -j flags
+echo /usr/bin/ninja $JFLAGS '"$@"' > /usr/local/bin/ninja
+chmod +x /usr/local/bin/ninja
+
+# Set MAKEFLAGS so that all make invocations in container builds include the
+# flags (doesn't apply to non-container builds, but we don't run make there)
+export MAKEFLAGS=$JFLAGS