Fix for modff() yielding wrong results

Detail:
  modff() single precision function yielded wrong results for both fractional
  and integral part for most numbers because the wrong exponent bias was subtracted.

c/math

@@ -946,7 +946,7 @@ float modff(float value, float *iptr)
         return value;
     }
     x.s = value;
-    if ((exponent = x.i.x - 0xff) < 0)
+    if ((exponent = x.i.x - 0x7f) < 0)
     {   *iptr = copysignf(0.0F, value);
         return value;
     }