[Bug libstdc++/113260] Add overloads of from_chars/to_chars for __float128

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113260

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I've changed the summary, because they're not "missing", since they're not
required/supposed to exist anyway.