Always use extern crate std in cranelift-codegen

cranelift/codegen/src/isa/mod.rs

@@ -25,7 +25,7 @@
 //! # fn main() {
 //! use cranelift_codegen::isa;
 //! use cranelift_codegen::settings::{self, Configurable};
-//! use std::str::FromStr;
+//! use alloc::str::FromStr;
 //! use target_lexicon::Triple;
 //!
 //! let shared_builder = settings::builder();
@@ -65,7 +65,7 @@ use crate::settings::SetResult;
 use crate::timing;
 use core::fmt;
 use failure_derive::Fail;
-use std::boxed::Box;
+use alloc::boxed::Box;
 use target_lexicon::{triple, Architecture, PointerWidth, Triple};
 
 #[cfg(feature = "riscv")]
@@ -119,7 +119,7 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
 /// Look for a supported ISA with the given `name`.
 /// Return a builder that can create a corresponding `TargetIsa`.
 pub fn lookup_by_name(name: &str) -> Result<Builder, LookupError> {
-    use std::str::FromStr;
+    use alloc::str::FromStr;
     lookup(triple!(name))
 }