x86_64: Add log1p with FMA

x86_64: Add log1p with FMA

On Skylake, it changes log1p bench performance by:

        Before       After     Improvement
max     63.349       58.347       8%
min     4.448        5.651        -30%
mean    12.0674      10.336       14%

The minimum code path is

 if (hx < 0x3FDA827A)                          /* x < 0.41422  */
    {
      if (__glibc_unlikely (ax >= 0x3ff00000))           /* x <= -1.0 */
        {
	   ...
        }
      if (__glibc_unlikely (ax < 0x3e200000))           /* |x| < 2**-29 */
        {
          math_force_eval (two54 + x);          /* raise inexact */
          if (ax < 0x3c900000)                  /* |x| < 2**-54 */
            {
	      ...
            }
          else
            return x - x * x * 0.5;

FMA and non-FMA code sequences look similar.  Non-FMA version is slightly
faster.  Since log1p is called by asinh and atanh, it improves asinh
performance by:

        Before       After     Improvement
max     75.645       63.135       16%
min     10.074       10.071       0%
mean    15.9483      14.9089      6%

and improves atanh performance by:

        Before       After     Improvement
max     91.768       75.081       18%
min     15.548       13.883       10%
mean    18.3713      16.8011      8%