bootstrap: use new check-cfg syntax when building stage 2 and above

This is required by the libc crate since it uses the new syntax as soon
as it detects rustc version 1.75 or above, which is the case when
building with stage 1 and above.

Author: Urgau

src/bootstrap/src/core/builder.rs

@@ -1404,7 +1404,7 @@ impl<'a> Builder<'a> {
         }
 
         // #[cfg(bootstrap)]
-        let use_new_check_cfg_syntax = self.local_rebuild;
+        let use_new_check_cfg_syntax = stage >= 1;
 
         // Enable compile-time checking of `cfg` names, values and Cargo `features`.
         //