[libstdc++] decimal float support (take 3)

[Benjamin Kosnik]

> I think that in the spirit of having tr1 components in a subdirectory 
> named after the namespace it might be better to have the decimal 
> extension under a decimal directory.  That name would make it more 
> recognizable for people and it would be just as "hidden".
> 
> Just a thought.

Much better, agreed.

I'm going to do these two things and check in, and then re-gen the docs.

-benjamin