libsubcmd: Use -O0 with DEBUG=1

When a 'make DEBUG=1' build is done, the command parser is still built
with -O6 and is hard to step through, fix it making it use -O0 in that
case.

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: nd <nd@arm.com>
Link: http://lore.kernel.org/lkml/20191028113340.4282-1-james.clark@arm.com


[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>