tracing: New flag to allow non privileged users to use a trace event

This adds a new trace event internal flag that allows them to be
used in perf by non privileged users in case of task bound tracing.

This is desired for syscalls tracepoint because they don't leak
global system informations, like some other tracepoints.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Jason Baron <jbaron@redhat.com>