Check debug targets of submodules

This adds another job to the `softload` stage, which ensures that the `debug`
target builds successfully.

Since some makefiles don't define such a target, we filter based on the
results of `make -n debug` first, to ensure that the only failures reported
are those generated by tools invoked by `make`.
1 job for debug-builds in 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Generate
passed #59658
cross
generate

00:00:04