* gc.c: add verifying counters code in gc_verify_internal_consistency().
  gc_verify_internal_consistency() counts all
  - live objects
  - young objects (if age2 promotion)
  - old objects
  in all pages and compares with objspace managing counters.
* gc.c (gc_after_sweep): do gc_verify_internal_consistency()
  when RGENGC_CHECK_MODE >= 2.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46335 b2dd03c8-39d4-4d8f-98ff-823fe69b080e