cc-wrapper: match useGccForLibs conditional order

This breaks an evaluation cycle between `cc` and `gccForLibs` for cross
builds, fixing an infinite recursion error.

pkgs/build-support/cc-wrapper/default.nix

@@ -299,10 +299,11 @@ stdenv.mkDerivation {
     # vs libstdc++, etc.) since Darwin isn't `useLLVM` on all counts. (See
     # https://clang.llvm.org/docs/Toolchain.html for all the axes one might
     # break `useLLVM` into.)
-    + optionalString (isClang && gccForLibs != null
+    + optionalString (isClang
                       && targetPlatform.isLinux
                       && !(stdenv.targetPlatform.useAndroidPrebuilt or false)
-                      && !(stdenv.targetPlatform.useLLVM or false)) ''
+                      && !(stdenv.targetPlatform.useLLVM or false)
+                      && gccForLibs != null) ''
       echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     ''