CLJS-1541: Self-host: Cannot require 'cljs.js using cljs.jar

This change updates things to mimic the handling of the defn macro
defined previously in the file. A result is that defmacro is
available in the standalone shipping cljs.jar and this then makes
it possible to (require 'cljs.js).

Author: mfikes

src/main/clojure/cljs/core.cljc

@@ -2924,4 +2924,5 @@
       (cons `defn decl)
       (core/list 'set! `(. ~name ~'-cljs$lang$macro) true))))
 
-#?(:cljs (set! (. defmacro -cljs$lang$macro) true))
+#?(:clj  (. (var defmacro) (setMacro))
+   :cljs (set! (. defmacro -cljs$lang$macro) true))