re-enable the after-gc-hook

re-enable the after-gc-hook

* libguile/gc.c (scm_gc): No need to take a mutex here.  Don't run the
  hook, the hook will run itself.
  (scm_c_register_gc_callback): New private helper, registers a callback
  the next time GC happens.
  (system_gc_callback): Guile's internal callback that runs
  scm_after_gc_c_hook, which itself queues a call to the after-gc-hook.
  (scm_storage_prehistory): Queue up a call to system_gc_callback.