librustc: Remove pub extern and priv extern from the language.

Place `pub` or `priv` on individual items instead.

src/test/run-fail/morestack2.rs

@@ -23,7 +23,7 @@ use std::task;
 mod rustrt {
     use std::libc;
 
-    pub extern {
+    extern {
         pub fn rust_get_argc() -> libc::c_int;
     }
 }