fix custom mir doc tests

Author: Lukas Markeffsky

library/core/src/intrinsics/mir.rs

@@ -60,7 +60,8 @@
 //!
 //! # Examples
 //!
-//! ```rust
+#![cfg_attr(bootstrap, doc = "```rust,compile_fail")]
+#![cfg_attr(not(bootstrap), doc = "```rust")]
 //! #![feature(core_intrinsics, custom_mir)]
 //!
 //! extern crate core;
@@ -291,7 +292,8 @@ define!(
     ///
     /// # Examples
     ///
-    /// ```rust
+    #[cfg_attr(bootstrap, doc = "```rust,compile_fail")]
+    #[cfg_attr(not(bootstrap), doc = "```rust")]
     /// #![feature(custom_mir, core_intrinsics)]
     ///
     /// extern crate core;