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

[B. Kosnik]

>This is also fine by me.  I wasn't sure if it was an issue or not.
>
>Does anyone know why make check-abi doesn't complain about it?

The link maps are probably exporting all basic_ios, basic_istream bits
right now, so adding symbols means that they probably are added in under
the old version names. 

I remarked to Paolo a bit ago that this was a problem. Instead, new
symbols should be checked to come in under the most recent version.

Also, missing symbols should be flagged as incompatible.

Also, all standard types should be instantiated and checked, in addition
to the library binary.

Keep in mind that check-abi is still an evolving target... it was
designed as a second-check for people doing work, and can definitely
stand some improvement. It's my hope that all this ABI stuff will become
less like reading tea leaves and more like catching the Muni J train:
mostly works, but some delays...

best,
benjamin