gc.c: atomical add

* gc.c (gc_before_sweep, gc_after_sweep): add to increase2 atomically.

* gc.c (gc_marks): adjust indent.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43045 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-09-26

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43046 b2dd03c8-39d4-4d8f-98ff-823fe69b080e