Fix memory leak in list_available_thread_groups

Fix memory leak in list_available_thread_groups

Commit

  C++ify osdata
  479f8de1b3b7e69ca8d557bbe9d843c7d1bc89c5

introduced a memory leak.  We allocate std::vectors and insert them in a
map, but never free them.  Instead, the map value type can be
std::vector objects directly.

gdb/ChangeLog:

	* mi/mi-main.c (list_available_thread_groups): Change map value
	type to std::vector.