Introduce SUPPRESS_JOBS CI variable

Sometimes, a job isn't expected to pass (most often `softload_gnu`, because
certain components don't support non-Norcroft toolchains). By defining this
variable in a `.gitlab-ci.yml` file, we can force any of the jobs to be
skipped. This only applies to submodule pipelines: it is not sensible to
skip a build phase for a superproject pipeline.
1 job for RISC_OSLib in 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Generate
passed #35178
cross
generate

00:00:03