[fortran, patch] Add Fortran 2003 IEEE intrinsic modules

[FX]

> That's a reasonable decision, but it is actually used at least ten times
> as much as everything else put together, possibly a hundred times as much.

I believe we are in pretty different parts of the community. Around me I rarely see it used, while people check for nans, infinities, and exception flags often. Also, aborting on certain floating-point exceptions is widely used as a debugging aid.

> However, it is used in the form of selecting hard underflow using a
> compilation option, and not within the program.  You certainly DO have
> targets where it would work, even dynamically within the program, and I
> think that it could be done even on x86.  That isn't the same as it
> should be done, of course!

Indeed, 387/SSE has flush-to-zero modes. But other APIs do not (glibc, SysV, AIX).
I’m perfectly willing to add it, especially to 387/SSE, if given a bit of help (someone to write the assembly code).

Thanks for your feedback,
FX