Refactor GStaticPrivate accessors to facilitate protecting them with locks

Refactor GStaticPrivate accessors to facilitate protecting them with locks

* g_static_private_get: have a single entry and exit

* g_static_private_set: delay creation of GArray so the whole tail of
  the function can be under the private_data lock without risking
  deadlock with the g_thread lock; call the destructor last, after
  we could have unlocked

* g_static_private_free: choose next thread in list before accessing
  private_data, to keep all accesses together

* g_thread_cleanup: steal private_data first, then work exclusively with
  the stolen array (which doesn't need to be under a lock any more)

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=642026
Bug-NB: NB#257512