PR fortran/23815: Add byte-swapping to gfortran

[François-Xavier Coudert]

> Currently, there is only one method of supplying the information
> of which files to byte-swap, via a CONVERT keyword on an OPEN
> statement.

Is that an already existent extension used by other compilers (and
which)? I am *strongly* against new extensions: we can't complain that
we have to support lots of stupid extensions that didn't age well
*and* introduce new ones. Most compilers allow to do that with a
compile-time (or link-time) switch; that's what i have read on most
tutorials about this problem in scientific codes. We have such a
possibility (I introduced it, see how the -ffpe-trap option is
handled).

FX