Fix libcore unit tests in stage 0

Author: SimonSapin

library/core/tests/lib.rs

@@ -68,7 +68,7 @@
 #![feature(option_result_unwrap_unchecked)]
 #![feature(option_unwrap_none)]
 #![feature(peekable_peek_mut)]
-#![feature(ptr_metadata)]
+#![cfg_attr(not(bootstrap), feature(ptr_metadata))]
 #![feature(once_cell)]
 #![feature(unsafe_block_in_unsafe_fn)]
 #![feature(unsized_tuple_coercion)]

library/core/tests/ptr.rs

@@ -1,5 +1,8 @@
 use core::cell::RefCell;
-use core::ptr::{self, *};
+#[cfg(not(bootstrap))]
+use core::ptr;
+use core::ptr::*;
+#[cfg(not(bootstrap))]
 use std::fmt::{Debug, Display};
 
 #[test]