perf pmu: Add flex debug build flag

perf pmu: Add flex debug build flag

Allow pmu parser's flex to be debugged as the parse-events and expr
currently are. Enabling this requires the C code to call
perf_pmu__flex_debug.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200619043356.90024-5-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>