libstdc++/9828: Regression: Segmentation fault in num_put::put

[Benjamin Kosnik]

The other advantage of this approach is that it's more extensible: we
could add more caches as we get the time, and not have to worry about
the size of __locale_cache changing, adding more accessor functions, and
link map maneuvering.

I'd been a bit worried about this, since large parts of the locale
facets haven't yet been caches (floating point numerics, everything
else).

-benjamin