zero-alloc cache (was Re: [v3] Fix PR libstdc++/10276)

[B. Kosnik]

>I've been stuck in the real world for a while, but I'm now looking at
>this again.  Then I'll get back to reworking the caches to be indexed
>by charT for a locale.

Great!

>I just tried on an Athlon with the current 3.3 branch and valgrind
>1.9.5 is not showing any leaks.  Very odd ...  The caches for the
>stdio objects should still leak, no?  I have to go back and check on
>the other machine.

I see the leak on mainline. 

-benjamin