feature: replace lazy_static by SyncLazy from std

compiler/rustc_feature/src/lib.rs

@@ -11,6 +11,8 @@
 //! even if it is stabilized or removed, *do not remove it*. Instead, move the
 //! symbol to the `accepted` or `removed` modules respectively.
 
+#![feature(once_cell)]
+
 mod accepted;
 mod active;
 mod builtin_attrs;