perf trace: Introduce function to set the base timestamp

That is used in both live runs, i.e.:

  # trace ls

As when processing events recorded in a perf.data file:

  # trace -i perf.data

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-901l6yebnzeqg7z8mbaf49xb@git.kernel.org


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