Add --print host-triple

People often parse `-vV` output to get to the host triple, which is
annoying to do. It's easier to just get it directly.

compiler/rustc_driver_impl/src/lib.rs

@@ -731,6 +731,7 @@ fn print_crate_info(
                 targets.sort_unstable();
                 println_info!("{}", targets.join("\n"));
             }
+            HostTuple => println_info!("{}", rustc_session::config::host_tuple()),
             Sysroot => println_info!("{}", sess.sysroot.display()),
             TargetLibdir => println_info!("{}", sess.target_tlib_path.dir.display()),
             TargetSpec => {