0.8.12.42

0.8.12.42
         Fix the threading problems revealed by Edi's CL-PPCRE tests ...

	 If target-arch.h is included before genesis/config.h, some
         runtime files with inline expansion of get_spinlock get the
         wrong version

	 Significantly change handling of thread exit: the
         SIG_THREAD_EXIT handler just makes th->state=STATE_DEAD, does
         not do all_threads surgery, does not call Lisp code and is no
         longer deferrable, eliminating a problem where threads die at
         the start of GC and become zombified so cannot be stopped for
         GC

	 Instead we call thread exit handlers from reap_dead_threads(),
         necessitating further changes in HANDLE-THREAD-EXIT, as it
	 can now be called from threads other than the parent of the
	 dead one

         stop_the_world doesn't actually need to hold
         all_threads_lock(), as it doesn't modify the all_threads
         list.  Likewise sig_stop_for_gc_handler(), which means the
         sched_yield() kludge can go away