PATCH: fix -fdump-ada-spec after PR debug/51410

[Eric Botcazou]

> Fixed by now taking into account ext_block in collect_all_refs and
> for_each_global_decl (and moving the declaration of ext_block up in the
> file).
>
> Tested on i686-pc-linux-gnu, OK for trunk?

Since it only affects -fdump-ada-spec, I took the liberty to backport it to the 
4.6 branch, as Jakub's patch has been installed there as well.

-- 
Eric Botcazou