ftrace: add tracing of context switches

This patch adds context switch tracing, of the format of:

                  _------=> CPU#
                 / _-----=> irqs-off
                | / _----=> need-resched
                || / _---=> hardirq/softirq
                ||| / _--=> preempt-depth
                |||| /
                |||||     delay
    cmd     pid ||||| time  |      pid:prio:state
       \   /    |||||   \   |      /
  swapper-0     1d..3    137us+:  0:140:R --> 2912:120
     sshd-2912  1d..3    216us+:  2912:120:S --> 0:140
  swapper-0     1d..3    261us+:  0:140:R --> 2912:120
     bash-2920  0d..3    267us+:  2920:120:S --> 0:140
     sshd-2912  1d..3    330us!:  2912:120:S --> 0:140
  swapper-0     1d..3   2389us+:  0:140:R --> 2847:120
 yum-upda-2847  1d..3   2411us!:  2847:120:S --> 0:140
  swapper-0     0d..3  11089us+:  0:140:R --> 3139:120
 gdm-bina-3139  0d..3  11113us!:  3139:120:S --> 0:140
  swapper-0     1d..3 102328us+:  0:140:R --> 2847:120
 yum-upda-2847  1d..3 102348us!:  2847:120:S --> 0:140

 "sched_switch" is added to /debugfs/tracing/available_tracers

[ Eugene Teo <eugeneteo@kernel.sg: remove unused tracing_sched_switch_enabled ]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>