gitlab-ci: Use 8 CPU cores in autotools job

This cuts down the job runtime from ~9.5 to ~7 minutes with my personal
runner on an 8-core Ryzen 7 1700.

While this might result in slightly higher load on shared runners, it
should be OK, since libtool doesn't use the CPU cores as effectively as
e.g. ninja does; a significant part of the CPU load tends to be in bash
processes at any time, which should be relatively light on memory.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci.yml

@@ -126,7 +126,7 @@ ubuntu:
 autotools:
   extends: .build
   variables:
-    MAKEFLAGS: "-j4"
+    MAKEFLAGS: "-j8"
     LLVM_CONFIG: llvm-config-7
   script:
     - mkdir build