tools lib traceevent: Add API to read time information from kbuffer

tools lib traceevent: Add API to read time information from kbuffer

Add the functions kbuffer_subbuf_timestamp() and kbuffer_ptr_delta() to
get the timing data stored in the ring buffer that is used to produced
the time stamps of the records.

This is useful for tools like trace-cmd to be able to display the
content of the read data to understand why the records show the time
stamps that they do.

Link: http://lore.kernel.org/linux-trace-devel/20200625100516.365338-2-tz.stoyanov@gmail.com
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[ Ported from trace-cmd.git ]
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200702185703.619656282@goodmis.org
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>