mk: Run doc tests with --cfg dox

There were a few examples in the macros::builtin module that weren't being run
because they were being #[cfg]'d out.

Closes #14697

src/libstd/macros.rs

@@ -465,7 +465,7 @@ pub mod builtin {
     /// ```
     /// let rust = bytes!("r", 'u', "st", 255);
     /// assert_eq!(rust[1], 'u' as u8);
-    /// assert_eq!(rust[5], 255);
+    /// assert_eq!(rust[4], 255);
     /// ```
     #[macro_export]
     macro_rules! bytes( ($($e:expr),*) => ({ /* compiler built-in */ }) )
@@ -482,10 +482,14 @@ pub mod builtin {
     /// # Example
     ///
     /// ```
+    /// #![feature(concat_idents)]
+    ///
+    /// # fn main() {
     /// fn foobar() -> int { 23 }
     ///
     /// let f = concat_idents!(foo, bar);
     /// println!("{}", f());
+    /// # }
     /// ```
     #[macro_export]
     macro_rules! concat_idents( ($($e:ident),*) => ({ /* compiler built-in */ }) )