Remove extern crate rustc_macros from numerous crates.

compiler/rustc_target/src/spec/mod.rs

@@ -40,6 +40,7 @@ use crate::json::{Json, ToJson};
 use crate::spec::abi::Abi;
 use crate::spec::crt_objects::CrtObjects;
 use rustc_fs_util::try_canonicalize;
+use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::symbol::{kw, sym, Symbol};
 use serde_json::Value;
@@ -51,8 +52,6 @@ use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::{fmt, io};
 
-use rustc_macros::HashStable_Generic;
-
 pub mod abi;
 pub mod crt_objects;