[Bug libstdc++/118855] Simplify when __builtin_*g builtins are available

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118855

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> I'll test this properly tomorrow, but it passes the 26_numerics/bit/* tests:

That is exactly what I meant.