NAG Fortran: Fix response file flag for creating archives

Since commit 10d6c3a635 (NAG: Pass response files through front-end to
the linker, 2018-08-01, v3.13.0-rc1~244^2) we use `-Wl,@` instead of `@`
in order to pass the response file through the `nagfor` compiler driver
to the linker.  However, this is not needed for the archiver.  Create
a separate rule variable so we can use different response file flags
for archiving and linking.

Issue: #23577