NAG Fortran: Use `-xldarg @` to pass response files to the linker

The `nagfor` compiler driver does not invoke the linker directly.
Instead it runs the system C compiler driver, which then invokes the
linker.  Passing `@` to NAG Fortran expands the response file before
it reaches the linker, which might lead to too long linker lines.
`-Wl,@`, used since commit 10d6c3a635 (NAG: Pass response files
through front-end to the linker, 2018-08-01, v3.13.0-rc1~244^2), can
lead to wrong placement of the response file on the linker line and
doesn't work at all when the response file is the only object file
passed to the NAG Fortran frontend.

Instead use `-xldarg @` to pass a response file through to the linker
without any intermediate expansion.  However, the response file
generated by Ninja can contain flags meant for the compiler driver,
so we need to disable response files for these flags altogether.

Fixes: #23577