perf tools: Pass tool context in the the perf_event_ops functions

So that we don't need to have that many globals.

Next steps will remove the 'session' pointer, that in most cases is
not needed.

Then we can rename perf_event_ops to 'perf_tool' that better describes
this class hierarchy.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-wp4djox7x6w1i2bab1pt4xxp@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/callchain.h

@@ -101,6 +101,9 @@ int callchain_append(struct callchain_root *root,
 int callchain_merge(struct callchain_cursor *cursor,
 		    struct callchain_root *dst, struct callchain_root *src);
 
+struct ip_callchain;
+union perf_event;
+
 bool ip_callchain__valid(struct ip_callchain *chain,
 			 const union perf_event *event);
 /*