Add SMP GC primitives and use them (with 1 processor).

Add SMP GC primitives and use them (with 1 processor).

Add SMP-GC-LOCK, SMP-GC-UNLOCK and SMP-GC-WAIT.  Add a stub for the
remaining unimplemented SMP primitive: SMP-LOCK-OBARRAY.  Make
SMP-COUNT return 1 so that the runtime system sets enable-smp? and
calls the new primitives.

Drop the threads_mutex (but keep threads_processor set) in SMP-GC-LOCK
so that processors waiting to enter the thread system (in
SMP-LOCK-THREADS) will wake, enter the GC-WAIT state, then try again.