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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.

Some worked examples:

Pipeline for a project with no softload stage at all (because it's type EXP in ModuleDB): https://gitlab.riscosopen.org/bavison/SyncLib/-/pipelines/4155

Pipeline for a project with a softload stage but where the makefile doesn't define a debug target (counts as success): https://gitlab.riscosopen.org/bavison/ConfigLib/-/pipelines/4167

Pipeline for a project with a debug target which builds successfully: https://gitlab.riscosopen.org/bavison/SDIODriver/-/pipelines/4169

Pipeline for a project with a debug target which fails (in this case because a dependency hasn't been merged yet): https://gitlab.riscosopen.org/bavison/SDIODriver/-/pipelines/4165