[gfortran] fortran preprocessing, round 2

[FX Coudert]

This is an updated version of my patch taking care of warnings when cc1 
is called from gfortran to preprocess fortran source. I used a 
different (and in my view, cleaner) approach: Fortran options are not 
marked as C, but when preprocessing fortran source, cc1 is given a 
-lang-fortran flag, which makes the preprocessor not warning about use 
of Fortran options.

Tested on i686-linux, with both languages=c and languages=c,fortran. 
The current behaviour for compiling C and Fortran in a single 
command-line is kept. Regtested for languages=c, regtesting in progress 
for languages=c,fortran -- howdy, how long is that C testsuite to run! 
:(

OK for mainline when it reopens before branching?

FX
:ADDPATCH fortran,c:

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