Always use extern crate std in cranelift-codegen

cranelift/codegen/src/ir/layout.rs

@@ -751,7 +751,7 @@ mod tests {
     use crate::entity::EntityRef;
     use crate::ir::{Ebb, Inst, ProgramOrder, SourceLoc};
     use core::cmp::Ordering;
-    use std::vec::Vec;
+    use alloc::vec::Vec;
 
     struct LayoutCursor<'f> {
         /// Borrowed function layout. Public so it can be re-borrowed from this cursor.