Allow building without std (#1069)

Closes https://github.com/CraneStation/cranelift/issues/1067

cranelift/frontend/src/frontend.rs

@@ -341,7 +341,7 @@ impl<'a> FunctionBuilder<'a> {
     /// This will not do anything unless `func.dfg.collect_debug_info` is called first.
     pub fn set_val_label(&mut self, val: Value, label: ValueLabel) {
         if let Some(values_labels) = self.func.dfg.values_labels.as_mut() {
-            use std::collections::hash_map::Entry;
+            use crate::hash_map::Entry;
 
             let start = ValueLabelStart {
                 from: self.srcloc,