GDB support for thread-local storage

[Andrew Cagney]

> Adding a suitable version symbol can only help, it can't break
>> anything. If it's there the debugger can use it. If not you can keep
>> doing what you already do, and it seems you really need it even for
>> the normal native cases.
> 
> 
> You'll never get them to document interface changes in pthreads'
> internal data structures.  You're welcome to try if you like pain more
> than I do.  The only layer the glibc folks support for accessing this
> data is thread_db.

Does the current thread-db code at least sanity check version match 
internally?

Andrew