gc.c: fixnum GC stress

* gc.c (garbage_collect_body): name magic numbers.

* gc.c (gc_stress_set): GC.stress accepts not only boolean but
  also Fixnum.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45644 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-21

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45645 b2dd03c8-39d4-4d8f-98ff-823fe69b080e