perf trace beauty clone: Handle CLONE_PIDFD

In addition to the older flags. This will allow something like this to
be implemented in 'perf trace"

  perf trace -e clone/PIDFD in flags/

I.e. ask for strace like tracing, system wide, looking for 'clone'
syscalls that have the CLONE_PIDFD bit set in the 'flags' arg.

For now we'll just see PIDFD if it is set in the 'flags' arg.

Cc: Christian Brauner <christian@brauner.io>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-drq9h7s8gcv8b87064fp6lb0@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/trace/beauty/clone.c

@@ -25,6 +25,7 @@ static size_t clone__scnprintf_flags(unsigned long flags, char *bf, size_t size,
 	P_FLAG(FS);
 	P_FLAG(FILES);
 	P_FLAG(SIGHAND);
+	P_FLAG(PIDFD);
 	P_FLAG(PTRACE);
 	P_FLAG(VFORK);
 	P_FLAG(PARENT);