Fix strtod ("nan") and strtold ("nan") returns wrong negative NaN

Fix strtod ("nan") and strtold ("nan") returns wrong negative NaN

The definition of qNaN for x86_64 and i386 was wrong.
strto{d|ld} ("nan") returned wrong negative NaN
instead of correct positive NaN
since it used the wrong definition.

On the other hand, strtof ("nan") returns correct positive NaN
since it uses nanf ("") instead of the wrong definition.

This commit makes strto{d|ld} ("nan") uses {nan|nanl} ("")
like strtof ("nan") using.
So strto{d|ld} ("nan") returns positive NaN.