llvm_12: add back --gcc-toolchain logic

In master commit 0710308 this was
removed from all LLVM versions, but we don't this commit on 20.09,
so when LLVM 12 was backported, there was no `--gcc-toolchain` in
`llvmPackages_12.stdenv`.

pkgs/development/compilers/llvm/12/default.nix

@@ -29,6 +29,8 @@ let
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/share" "$rsrc/share"
       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 {