1.0.41.38: ppc: Runtime damage for threads.

1.0.41.38: ppc: Runtime damage for threads.

  * Call out to pthread_getspecific() from call_into_lisp in
order to obtain the TLS block for lisp operation.

  * Tie together the runtime / gencgc version of pseudo-atomic
and the arch-specific / interrupt context version of pseudo-
atomic, to fulfill the requirements of the earlier pseudo-
atomic restructuring.

  * Generally use the thread structure instead of global
variables where appropriate.

  * Save our TLS block in the pthread per-thread storage area.

  * SIG_STOP_FOR_GC appears to default to masked on PPC, so
unblock it in initial_thread_trampoline().

  * Link with -lpthread when appropriate.