proc.c: original arity

* proc.c (rb_mod_method_arity): return original arity of the method if
  aliased because of visibility change, like as Method#arity.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38967 b2dd03c8-39d4-4d8f-98ff-823fe69b080e