make-derivation.nix: minor

it seems to change nothing (.platform is always there), just to be consisted with the style of other checks

pkgs/stdenv/generic/make-derivation.nix

@@ -253,7 +253,7 @@ in rec {
           enableParallelChecking = attrs.enableParallelChecking or true;
         } // lib.optionalAttrs (hardeningDisable != [] || hardeningEnable != []) {
           NIX_HARDENING_ENABLE = enabledHardeningOptions;
-        } // lib.optionalAttrs (stdenv.hostPlatform.isx86_64 && stdenv.hostPlatform.platform ? gcc.arch) {
+        } // lib.optionalAttrs (stdenv.hostPlatform.isx86_64 && stdenv.hostPlatform ? platform.gcc.arch) {
           requiredSystemFeatures = attrs.requiredSystemFeatures or [] ++ [ "gccarch-${stdenv.hostPlatform.platform.gcc.arch}" ];
         } // lib.optionalAttrs (stdenv.buildPlatform.isDarwin) {
           inherit __darwinAllowLocalNetworking;