Use fake libc in core test

Author: ChrisDenton

library/core/src/primitive_docs.rs

@@ -537,9 +537,10 @@ impl () {}
 /// ## 4. Get it from C.
 ///
 /// ```
-/// # #![feature(rustc_private)]
-/// #[allow(unused_extern_crates)]
-/// extern crate libc;
+/// # mod libc {
+/// # fn malloc(size: usize) -> *mut u8 { core::ptr::NonNull::dangling().as_ptr() }
+/// # fn free(mem: *mut u8) {}
+/// # }
 ///
 /// use std::mem;
 ///