THREAD_GETMEM support for Hurd in glibc

[Florian Weimer]

Is there a way to add fields to the thread descriptor on Hurd?

With NPTL, we have THREAD_GETMEM and THREAD_SETMEM to access
thread-local variables without relying on the platform TLS
implementation.

I want to use this for thread-local data in the dynamic loader,
specifically for the exception handling context.  Using the thread
descriptor or TCB for this seems appropriate because this TLS data is
not supposed to be duplicated in alternative namespaces (because they
share the dynamic loader).