Fortran: Use non conflicting file extensions for intermediates [PR81615]
gcc/ChangeLog:
* doc/invoke.texi: Document the new file extensions
gcc/fortran/ChangeLog:
PR fortran/81615
* lang-specs.h (F951_CPP_OPTIONS): Do not hardcode ".f90" extension
(F951_CPP_EXTENSION): Use .fi/.fii for fixed/free form sources
* options.cc (form_from_filename): Handle the new extensions