Do not use spin locks if AO test-and-set is emulated (pthreads)

Do not use spin locks if AO test-and-set is emulated (pthreads)

* include/private/gc_locks.h [BASE_ATOMIC_OPS_EMULATED && GC_PTHREADS]
(USE_PTHREAD_LOCKS): Define.
* include/private/gc_locks.h [BASE_ATOMIC_OPS_EMULATED && GC_PTHREADS]
(USE_SPIN_LOCK): Undefine.
* pthread_support.c [(USE_SPIN_LOCK || !NO_PTHREAD_TRYLOCK)
&& AO_CLEAR] (GC_pause): Do not use AO_compiler_barrier() if
BASE_ATOMIC_OPS_EMULATED.
* pthread_support.c [AO_HAVE_char_load] (is_collecting): Do not use
AO_char_load() if BASE_ATOMIC_OPS_EMULATED.