intern.h: inline function rb_check_arity

* include/ruby/intern.h (rb_check_arity): make a static inline
  function so it can be used as an expression and argc would be
  evaulated only once.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39598 b2dd03c8-39d4-4d8f-98ff-823fe69b080e