clang, cc-wrapper: Move --gcc-toolchain logic into CC wrapper

Take 2, after #94582 had to be reverted.

This reverts commit ac03cfa3c5.

pkgs/development/compilers/llvm/6/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {