[v3] system_error, round one

[Benjamin Kosnik]

> In practice, the issue is even less serious than I thought. Actually, 
> for char and wchar_t the function is only called by other functions 
> exported by the library anyway, thus redundant copies cannot happen in 
> user binaries. Still inside the .so there are *two* ("t"-type) copies of 
> __verify_grouping...

It looks like it might be inlinable, although I've not done anything 
other than eyeballed it.

If it's not inlined, this should be moved to locale_facets.cc.

-benjamin