perf trace: Handle unpaired raw_syscalls:sys_exit event

perf trace: Handle unpaired raw_syscalls:sys_exit event

Which may happen when we start a tracing session and a thread is waiting
for something like "poll" to return, in which case we better print "?"
both for the syscall entry timestamp and for the duration.

E.g.:

Tracing existing mutt session:

  # perf trace -p `pidof mutt`
          ? (     ?   ): mutt/17135  ... [continued]: poll()) = 1
      0.027 ( 0.013 ms): mutt/17135 read(buf: 0x7ffcb3c42cef, count: 1) = 1
      0.047 ( 0.008 ms): mutt/17135 poll(ufds: 0x7ffcb3c42c50, nfds: 1, timeout_msecs: 1000) = 1
      0.059 ( 0.008 ms): mutt/17135 read(buf: 0x7ffcb3c42cef, count: 1) = 1
  <SNIP>

Before it would print a large number because we'd do:

  ttrace->entry_time - trace->base_time

And entry_time would be 0, while base_time would be the timestamp for
the first event 'perf trace' reads, oops.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Claudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-wbcb93ofva2qdjd5ltn5eeqq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>