[Bug libstdc++/104870] [12 Regression] fast_float doesn't work for 16-bit size_t, but is used anyway by floating_from_chars.cc

[dv at vollmann dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104870

--- Comment #3 from dv at vollmann dot ch ---
(In reply to Jonathan Wakely from comment #1)
> Ah that #if should match the one in <charconv>

Yes, that's better than my patch.