tracing: Add trace_clock=<clock> kernel parameter

Being able to change the trace clock at boot can be advantageous if
you need a better source of when things happen across CPUs. The default
trace clock is the fastest, but it uses local clocks which may not be
synced across CPUs and it does not let you know when events took place
with respect to events on other CPUs.

The global trace clock can help in this case, and if you do not care
about timings, the counter "clock" is the best, as that is just a  simple
atomic counter that is incremented for every event.

Usage is to add "trace_clock=counter" on the kernel command line. You
can replace counter with "global" or any of the clocks listed in
/sys/kernel/debug/tracing/trace_clock

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Appreciated-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>