tracing: Add "perf" trace_clock

The function trace_clock() calls "local_clock()" which is exactly
the same clock that perf uses. I'm not sure why perf doesn't call
trace_clock(), as trace_clock() doesn't have any users.

But now it does. As trace_clock() calls local_clock() like perf does,
I added the trace_clock "perf" option that uses trace_clock().

Now the ftrace buffers can use the same clock as perf uses. This
will be useful when perf starts reading the ftrace buffers, and will
be able to interleave them with the same clock data.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>