rtld - Support static TLS bindings for late-loaded shared libraries

rtld - Support static TLS bindings for late-loaded shared libraries

* Allow late (manual) dlopen()s to load shared libraries which
  use static TLS variables, as long as there is space.  Do proper
  late-binding and initialize the area for all threads.

* rtld will cache a symbol lookup on first-need for:
  "_pthread_distribute_static_tls" and then call it as needed to
  initialize late-bound static TLS space.

  This symbol is weakly bounded to __libc_distribute_static_tls in libc,
  and strongly overridden by _libthread_distribute_static_tls in
  libthread_xu.

* Fixes mesa glx-tls and others.

* Test code from FreeBSD.  Also tested with other combinations
  including a pthread_create() and -static compilation.

	https://github.com/dumbbell/test-tls-initial-exec