What to do with argument mismatches in Fortran (was: [patch, fortran] Fix PR 91443)

[Thomas Koenig]

I wrote:

> Committed as r274551.

Well, this revision appears to have woken quite a few bugs from their
slumber.  While argument mismatch was always illegal, it seems to have
been a common idiom at one time.  And, like almost all bad habits of
the past, SPEC also has this (see PR 91473, where you can see thatt a
rather large number of SPEC tests now require -std=legacy).

So, what to do?  Is -std=legacy the right option, or should we add
something different (like -faccept-argument-mismatch), which we
could then set by -std=legacy?  And is there anything special
we should be doing after we have diagnoses the problem, if the
user simply wants to run the code?