perf test: fix failing test cases on linux-next for s390

In linux-next tree the many test cases fail on s390x when running the
perf test suite, sometime the perf tool dumps core.

Output before:
  6.1: Test event parsing                               : FAILED!
 10.3: Parsing of PMU event table metrics               : FAILED!
 10.4: Parsing of PMU event table metrics with fake PMUs: FAILED!
 17: Setup struct perf_event_attr                       : FAILED!
 24: Number of exit events of a simple workload         : FAILED!
 26: Object code reading                                : FAILED!
 28: Use a dummy software event to keep tracking        : FAILED!
 35: Track with sched_switch                            : FAILED!
 42.3: BPF prologue generation                          : FAILED!
 66: Parse and process metrics                          : FAILED!
 68: Event expansion for cgroups                        : FAILED!
 69.2: Perf time to TSC                                 : FAILED!
 74: build id cache operations                          : FAILED!
 86: Zstd perf.data compression/decompression           : FAILED!
 87: perf record tests                                  : FAILED!
106: Test java symbol                                   : FAILED!

The reason for all these failure is a missing PMU. On s390x the PMU is
named cpum_cf which is not detected as core PMU.  A similar patch was
added before, see commit 9bacbced ("perf list: Add s390 support
for detailed PMU event description") which got lost during the recent
reworks. Add it again.

Output after:
 10.2: PMU event map aliases                            : FAILED!
 42.3: BPF prologue generation                          : FAILED!

Most test cases now work and there is not core dump anymore.
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/20230616081437.1932003-1-tmricht@linux.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/pmu.c

@@ -1419,7 +1419,7 @@ void perf_pmu__del_formats(struct list_head *formats)
 
 bool is_pmu_core(const char *name)
 {
-	return !strcmp(name, "cpu") || is_sysfs_pmu_core(name);
+	return !strcmp(name, "cpu") || !strcmp(name, "cpum_cf") || is_sysfs_pmu_core(name);
 }
 
 bool perf_pmu__supports_legacy_cache(const struct perf_pmu *pmu)