* gc.c: refactoring.
* gc.c: use GC_ENABLE_INCREMENTAL_MARK instead of USE_RINCGC.
* gc.c (gc_start): check FORCE_MAJOR_GC.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48414 b2dd03c8-39d4-4d8f-98ff-823fe69b080e