.gdbinit: follow classname() change

* .gdbinit (rb_classname): follow classname() change at r36584, which
  hash second argument now.  reported by 36584 via IRC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42722 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
.gdbinit: improve for displaying class/module

* .gdbinit (rp_class): show class name, if possible.

* .gdbinit (rb_ancestors): call rp_class directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42723 b2dd03c8-39d4-4d8f-98ff-823fe69b080e