Fix a number of warnings on nightly Rust (#2652)

This fixes some issues that are cropping up where some syntax will get
phased out in 2021

cranelift/codegen/meta/src/isa/riscv/recipes.rs

@@ -25,7 +25,8 @@ impl RecipeGroup {
     fn push(&mut self, builder: EncodingRecipeBuilder) {
         assert!(
             self.name_to_recipe.get(&builder.name).is_none(),
-            format!("riscv recipe '{}' created twice", builder.name)
+            "riscv recipe '{}' created twice",
+            builder.name
         );
         let name = builder.name.clone();
         let number = self.recipes.push(builder.build());

cranelift/codegen/meta/src/isa/x86/encodings.rs

@@ -45,10 +45,8 @@ impl PerCpuModeEncodings {
         if let Some(found_index) = self.recipes_by_name.get(&recipe.name) {
             assert!(
                 self.recipes[*found_index] == recipe,
-                format!(
-                    "trying to insert different recipes with a same name ({})",
-                    recipe.name
-                )
+                "trying to insert different recipes with a same name ({})",
+                recipe.name
             );
             *found_index
         } else {