* gc.c (gc_stat_internal): rename `heap_used' to `heap_allocated_pages'.
  ref: [Feature #9924]
* test/ruby/test_gc.rb: add constraints test for gc stat information.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47493 b2dd03c8-39d4-4d8f-98ff-823fe69b080e