perf trace: Handle multiple threads better wrt syscalls being intermixed

perf trace: Handle multiple threads better wrt syscalls being intermixed

 $ trace time taskset -c 0 usleep 1
   0.845 ( 0.021 ms): time/16722 wait4(upid: 4294967295, stat_addr: 0x7fff17f443d4, ru: 0x7fff17f44438 ) ...
   0.865 ( 0.008 ms): time/16723 execve(arg0: 140733595272004, arg1: 140733595272720, arg2: 140733595272768, arg3: 139755107218496, arg4: 7307199665339051828, arg5: 3) = -2
   2.395 ( 1.523 ms): taskset/16723 execve(arg0: 140733595272013, arg1: 140733595272720, arg2: 140733595272768, arg3: 139755107218496, arg4: 7307199665339051828, arg5: 3) = 0
   2.411 ( 0.002 ms): taskset/16723 brk(                                                                  ) = 0x1915000
   3.300 ( 0.058 ms): usleep/16723 nanosleep(rqtp: 0x7ffff4ada190                                        ) = 0
 <SNIP>
   3.305 ( 0.000 ms): usleep/16723 exit_group(
   3.363 ( 2.539 ms): time/16722  ... [continued]: wait4()) = 16723
   3.366 ( 0.001 ms): time/16722 rt_sigaction(sig: INT, act: 0x7fff17f44160, oact: 0x7fff17f44200, sigsetsize: 8) = 0

We we're not seeing this line:

  0.845 ( 0.021 ms): time/16722 wait4(upid: 4294967295, stat_addr: 0x7fff17f443d4, ru: 0x7fff17f44438 ) ...

just the one when it finishes:

  3.363 ( 2.539 ms): time/16722  ... [continued]: wait4()) = 16723

Still some issues left till we move to ordered_samples when multiple
CPUs/threads are involved...

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-zq9x30a1ky3djqewqn2v3ja3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>