perf script: Fix --per-event-dump for auxtrace synth evsels

perf script: Fix --per-event-dump for auxtrace synth evsels

When processing PERF_RECORD_AUXTRACE_INFO several perf_evsel entries
will be synthesized and inserted into session->evlist, eventually ending
in perf_script.tool.sample(), which ends up calling builtin-script.c's
process_event(), that expects evsel->priv to be a perf_evsel_script
object with a valid FILE pointer in fp.

So we need to intercept the processing of PERF_RECORD_AUXTRACE_INFO and
then setup evsel->priv for these newly created perf_evsel instances, do
it to fix the segfault in process_event() trying to use a NULL for that
FILE pointer.

Reported-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: yuzhoujian <yuzhoujian@didichuxing.com>
Fixes: a14390fde64e ("perf script: Allow creating per-event dump files")
Link: http://lkml.kernel.org/n/tip-bthnur8r8de01gxvn2qayx6e@git.kernel.org
[ Merge fix by Ravi Bangoria before pushing upstream to preserv bisectability ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>