[PATCH, fortran]: fix a few more libgfortran warnings

[Tobias Burnus]

Ben Elliston wrote:
> This patch eliminates some warnings in intrinsics/signal.c due to some
> function parameters being unused depending on how the functions are
> compiled -- ie. in the absence of SIGALRM, alarm() and signal() on the
> target.
>
> Okay for mainline?
>   
OK.

Tobias