Mark used-in-subtree-only cg_gcc field as #[allow(unused)]

This field will be used in future commits currently in the subtree and
should not be removed.

Author: krtab

compiler/rustc_codegen_gcc/build_system/src/config.rs

@@ -5,6 +5,7 @@ use std::env as std_env;
 pub struct ConfigInfo {
     pub target: String,
     pub target_triple: String,
+    #[allow(dead_code)]
     pub rustc_command: Vec<String>,
 }