[committed] libstdc++: Add std::from_chars for floating-point types

[Florian Weimer]

* Jonathan Wakely via Libstdc:

> By replacing the use of strtod we could avoid allocation, avoid changing
> locale, and use optimised code paths specific to each std::chars_format
> case. We would also get more portable behaviour, rather than depending
> on the presence of uselocale, and on any bugs or quirks of the target
> libc's strtod. Replacing strtod is a project for a later date.

glibc already has strtod_l (since glibc 2.1, undocumented, but declared
in <stdlib.h>).

What seems to be missing is a function that takes an explicit buffer
length.  A static reference to the C locale object would be helpful as
well, I assume.

Maybe this is sufficiently clean that we can export this for libstdc++'s
use?  Without repeating the libio mess?

Thanks,
Florian