firefox: fixup build on aarch64-linux

It's the issue with old libgcc_s propagated via our glibc package; e.g.
https://github.com/NixOS/nixpkgs/pull/209113

pkgs/applications/networking/browsers/firefox/common.nix

@@ -471,6 +471,8 @@ buildStdenv.mkDerivation ({
   separateDebugInfo = enableDebugSymbols;
   enableParallelBuilding = true;
 
+  NIX_LDFLAGS = if (with stdenv; isAarch64 && isLinux) then [ "-lgcc" ] else null;
+
   # tests were disabled in configureFlags
   doCheck = false;