[gfortran] Fix NEAREST constant folder

[Tobias Schlüter]

Steve Kargl wrote:
> I sent a message off to the MPFR list to ask about the
> future of subnormal numbers.  Still waiting for a reply.
> MPFR ensures a correctly rounded significand.  The
> exponent in principle has an unlimited range, so wrapping
> MPFR should be possible.

One can limit the exponent range via the mpfr_set_emin and mpfr_set_emax
functions, but after playing around with them for a minute it looks like that
x < 2**emin gets rounded to zero in all cases, so we might indeed have to wrap
this some way or another.

- Tobi