Always use extern crate std in cranelift-codegen

cranelift/codegen/src/isa/x86/mod.rs

@@ -18,7 +18,7 @@ use crate::regalloc;
 use crate::result::CodegenResult;
 use crate::timing;
 use core::fmt;
-use std::boxed::Box;
+use alloc::boxed::Box;
 use target_lexicon::{PointerWidth, Triple};
 
 #[allow(dead_code)]

cranelift/codegen/src/isa/x86/registers.rs

@@ -8,7 +8,7 @@ include!(concat!(env!("OUT_DIR"), "/registers-x86.rs"));
 mod tests {
     use super::*;
     use crate::isa::RegUnit;
-    use std::string::{String, ToString};
+    use alloc::string::{String, ToString};
 
     #[test]
     fn unit_encodings() {