perf test sigtrap: Print errno string when failing

Helps a bit the user figuring out why it is failing:

Before:

  $ perf test sigtrap
  73: Sigtrap                                                         : FAILED!
  $ perf test -v sigtrap
  73: Sigtrap                                                         :
  --- start ---
  test child forked, pid 3816772
  FAILED sys_perf_event_open()
  test child finished with -1
  ---- end ----
  Sigtrap: FAILED!
  $

After:

  $ perf test sigtrap
  73: Sigtrap                                                         : FAILED!
  $ perf test -v sigtrap
  73: Sigtrap                                                         :
  --- start ---
  test child forked, pid 3816772
  FAILED sys_perf_event_open(): Permission denied
  test child finished with -1
  ---- end ----
  Sigtrap: FAILED!
  $

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Fabian Hemmer <copy@copy.sh>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kasan-dev@googlegroups.com
Link: http://lore.kernel.org/lkml/YZOpSVOCXe0zWeRs@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>