[Bug libstdc++/70299] pow(long double, int) gives more imprecise result than pow(long double,long double) in c++03 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70299

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Using the __builtin_powi* functions was done for PR11706, I believe because
they can be a lot more efficient than the general powl(double, double)
implementations. So it seems that for C++11 and later we provide more accuracy,
but probably via slower functions than we use for C++03.