gc.c: refine macros

* gc.c (MARKED_IN_BITMAP, FL_TEST2): return boolean value since always
  used as boolean value.
* gc.c (MARK_IN_BITMAP, CLEAR_IN_BITMAP): evaluate bits once.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41445 b2dd03c8-39d4-4d8f-98ff-823fe69b080e