Fix sign of exact zero return from fma (bug 14638)

[David Miller]

From: "Joseph S. Myers" <joseph@codesourcery.com>
Date: Fri, 28 Sep 2012 20:05:40 +0000

> Tested x86_64 and x86.  I'd welcome testing for an ldbl-128
> architecture with rounding mode support, such as sparc, to verify that
> I did find the right places to change for ldbl-128 (the ldbl-128
> target most convenient for me to test on myself is mips64, which
> doesn't have rounding modes or exceptions for long double).

I'll do some sparc testing for you when I get a chance.