Basically implementing what Andrew said in the PR:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77576

If @file has been passed to gcc-ar, do the following:

1) Expand it to get an argv without any @files.
2) Then apply the plugin modifications to argv.
3) Create temporary response file.
4) Put the modified argv in the temporary file.
5) Call ar with @tmp.
6) Delete the temporary response file.