The nextafter functions use the wrong next representable value

From C99 standard: “The nextafter functions determine the next representable value, in the type of the function,
after x in the direction of y, where x and y are first converted to the type of the function”.
The next representable value of 0.0 in direction of -1.0 is -4.9406564584124654e-324, not 0.0.

Similar thing holds for nextafterf, nextafterl, nexttowardf, nexttoward, and nexttowardl.
The tests pass either way, since the error is within the tolerance, but how it is written is wrong.

Change-Id: I1338eeffe3de8031a48f46e1b07146bc07dc2f0a
Signed-off-by: Jingwei Zhang <jingwei.zhang@intel.com>
Signed-off-by: Mingwei Shi <mingwei.shi@intel.com>