compilers: unify fortran sanity check with its parent Clike handling

We *mostly* just need to do the same thing. Plug in one utility method
to make sanity_check_impl find the right compile args, and plug in
DEVNULL to the test run. It's that simple.

This solves a few inconsistencies. The main one is that fortran never
logged the sanity checks to the Meson debug log, making it hard to
debug.

There's also some interesting quirks we built up in the dedicated
fortran handling. For example:

- in commit 5b109c9ad27aea39ce49d1da8ef0c957ccaef3b9 we added cwd to
  building the fortran executable, with a wordy comment about how the
  compiler has defects. But the clike base has always done that on
  general principle anyway, so we would never have had that bug in the
  first place.

- in commit d6be7822a0a7391c9d2a22c053cd4fc61b5a71e4 we added special
  deletion of an old "bad existing exe file" just for fortran. Looking
  at the PR discussion for this odd requirement, it turns out that the
  real problem is mixing WSL and native Windows without deleting the
  build directory. This is apparently fortran specific simply because
  "contemporary Windows 10 Fortran users" switch between the two?

  The actual problem is that this never used .exe as the output name, so
  Windows thinks you want to run something other than the thing you
  asked to run, because it's not even a Window executable. But... the
  common clike handling could have fixed that without needing special
  cases.

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