xtask: add a hint to install cargo binutils

Author: luojia65

xtask/src/main.rs

@@ -308,6 +308,7 @@ fn check_tool<S: AsRef<str>>(tool: S) -> Option<String> {
             return Some(format!("riscv64-unknown-elf-{}", tool.as_ref()));
         }
     }
+    println!("No binutils found, try install using `cargo install cargo-binutils`.");
     return None;
 }