gc.c: use switch for alternative BUILTIN_TYPE

* gc.c (rb_objspace_call_finalizer): BUILTIN_TYPE is alternative,
  T_DATA object (Thread, Mutex, Fiber) cannot be T_FILE.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43576 b2dd03c8-39d4-4d8f-98ff-823fe69b080e