Suggest gcc_multi to make mir_opts run on nixos (rust-lang#1841)

Author: Nadrieril

src/doc/rustc-dev-guide/src/building/suggested.md

@@ -328,7 +328,7 @@ in
 pkgs.mkShell {
   name = "rustc";
   nativeBuildInputs = with pkgs; [
-    gcc9 binutils cmake ninja openssl pkgconfig python39 git curl cacert patchelf nix psutils
+    gcc_multi binutils cmake ninja openssl pkgconfig python39 git curl cacert patchelf nix psutils
   ];
   RIPGREP_CONFIG_PATH = ripgrepConfig;
   RUST_BOOTSTRAP_CONFIG = config;