* gc.c (gc_stress_set): add special option of GC.stress.
  `GC.stress=(flag)' accepts integer to control behavior of GC.
  See code for details. Of course, this feature is only for MRI.
  You can debug RGenGC (WB) using `GC.stress = 1'.
  Using this option, do minor marking at all possible places.
  GC::STRESS_MINOR_MARK = 1 and GC::STRESS_LAZY_SWEEP = 2
  seem good to add.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41400 b2dd03c8-39d4-4d8f-98ff-823fe69b080e