perf tools: Allocate thread map_groups's dynamically

Moving towards sharing map groups within a process threads.

Because of this we need the map groups to be dynamically allocated. No
other functional change is intended in here.

Based on a patch by Jiri Olsa, but this time _just_ making the
conversion from statically allocating thread->mg to turning it into a
pointer and instead of initializing it at thread's constructor,
introduce a constructor/destructor for the map_groups class and
call at thread creation time.

Later we will introduce the get/put methods when we move to sharing
those map_groups, when the get/put refcounting semantics will be needed.

Signed-off-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1397490723-1992-3-git-send-email-jolsa@redhat.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/util/thread.h

@@ -13,7 +13,7 @@ struct thread {
 		struct rb_node	 rb_node;
 		struct list_head node;
 	};
-	struct map_groups	mg;
+	struct map_groups	*mg;
 	pid_t			pid_; /* Not all tools update this */
 	pid_t			tid;
 	pid_t			ppid;