libmvec in gcc to have vector math in fortran

[Florian Weimer]

* Richard Biener:

> That said, good to see some glibc folks jump in on this thread.
> I'd like to see glibc provide a fortran intrinsic header advertising
> the libmvec routines it has.  We probably do need some gfortran
> adjustments here but I think that glibc advertising is better than
> hard-coding lists into GCC (like we do for some targets with
> their -mveclibabi= option).

The advantage of putting into GCC is that it will work irrespective of
the installed glibc header version, so if we get this going:

  <https://sourceware.org/ml/libc-alpha/2018-05/msg00501.html>

you could use libmvec along with really old glibcs (probably as far
back as 2.12, maybe even older).

But then we might into a different direction altogether.

Another question is wheter the Fortran header can be generated by
something like -fdump-ada-spec, so that we don't have to maintain it
separately from the C header.