tracing: Use dyn_event framework for synthetic events

Use dyn_event framework for synthetic events. This shows
synthetic events on "tracing/dynamic_events" file in addition
to tracing/synthetic_events interface.

User can also define new events via tracing/dynamic_events
with "s:" prefix. So, the new syntax is below;

  s:[synthetic/]EVENT_NAME TYPE ARG; [TYPE ARG;]...

To remove events via tracing/dynamic_events, you can use
"-:" prefix as same as other events.

Link: http://lkml.kernel.org/r/154140861301.17322.15454611233735614508.stgit@devbox

Reviewed-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/Kconfig

@@ -635,6 +635,7 @@ config HIST_TRIGGERS
 	depends on ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select TRACING_MAP
 	select TRACING
+	select DYNAMIC_EVENTS
 	default n
 	help
 	  Hist triggers allow one or more arbitrary trace event fields