1.0.25.29: thread state visibility and synchronization

1.0.25.29: thread state visibility and synchronization

C does not guarantee that changes made to a volatile variable in one
thread are visibile to other threads. Use locking primitives (that
have memory barrier semantics) for that purpose.

Thus, all_threads need not be volatile: it's always accessed while
holding all_threads_lock. Thread state does not need volatile either,
as signal a handlers don't change it (save for sig_stop_for_gc_handler
but that sets it restores its value). But visibility issues can arise
and potentially deadlock in stop_the_world, so the thread state is
given a lock. And to reduce busy looping while waiting for the state
to change to STATE_SUSPENDED a condition variable is added as well.

Also convert sig_stop_for_gc_handler to use
wait_for_thread_state_change which frees up SIG_RESUME_FROM_GC. I
think this also guarantees that the changes made by the gc are visible
in other threads on non x86 platforms (on x86 it's already the case).

With these changes threads.impure.lisp runs 10% a faster in real time.