gfortran: use version 8 by default to match our C and C++ compiler

We recently updated to gcc 8.x, but our default Fortran compiler is still at
version 7.x. This causes trouble for all packages that mix C++ and Fortran
code, like R does.

Unfortunately, gfortran8 is marked broken on Darwin, so we cannot switch on
that platform yet:

 | $ nix-instantiate --arg nixpkg '{ outPath = ./.; revCount = 999999; shortRev = "peti"; }' ./pkgs/top-level/release.nix -A unstable --show-trace
 | [...]
 | while evaluating the attribute 'handled' at /home/simons/src/nixpkgs/pkgs/stdenv/generic/check-meta.nix:251:7:
 | while evaluating 'handleEvalIssue' at /home/simons/src/nixpkgs/pkgs/stdenv/generic/check-meta.nix:147:38, called from /home/simons/src/nixpkgs/pkgs/stdenv/generic/check-meta.nix:252:14:
 | Failed to evaluate gfortran-wrapper-8.3.0: «broken»: is marked as broken