qt5.qtwebengine, webkitgtk: try harder to avoid timeouts

I hope this will improve the situation on aarch64-linux.
I don't think it could make anything worse.
Recent staging-next timeouts:
https://hydra.nixos.org/build/141551270
https://hydra.nixos.org/build/141547098

For now let's not touch meta.timeout, as there's less consensus and the
setting seems less reliable in practice, too.

pkgs/development/libraries/qt-5/modules/qtwebengine.nix

@@ -236,6 +236,8 @@ qtModule {
     sed 's/${lib.head (lib.splitString "-" version)} /${qtCompatVersion} /' -i "$out"/lib/cmake/*/*Config.cmake
   '';
 
+  requiredSystemFeatures = [ "big-parallel" ];
+
   meta = with lib; {
     description = "A web engine based on the Chromium web browser";
     maintainers = with maintainers; [ matthewbauer ];

pkgs/development/libraries/webkitgtk/default.nix

@@ -177,6 +177,8 @@ stdenv.mkDerivation rec {
     patchShebangs .
   '';
 
+  requiredSystemFeatures = [ "big-parallel" ];
+
   meta = {
     description = "Web content rendering engine, GTK port";
     homepage = "https://webkitgtk.org/";