`aarch64-unknown-hermit` std build break

[Pistonight]

building std for aarch64-unknown-hermit produces this error:

error: `#[stable]` attribute cannot be used on extern crates
 --> library/std/src/os/hermit/mod.rs:5:1
5 | #[stable(feature = "rust1", since = "1.0.0")]

The error is on the hermit_abi extern crate declaration

pub extern crate hermit_abi;

The command I tried is ./x --stage 1 build library

Appears to be related to 35e04b6

[jieyouxu]

cc @JonathanBrouwer

[JonathanBrouwer]

@rustbot claim
should be an easy fix, will fix it sometime in the next few days

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-low -regression-untriaged