__builtin_isinf_sign doesn't reflect the correct return value for NaN
to emulate finite function. Use __builtin_isfinite instead whichg is available
since GCC 4.4 just as __builtin_isinf_sign.
* libm/common/sl_finite.c (finitel): Use __builtin_isfinite.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>