Do not use 'ifndef AO_CLEAR' in mark, pthread_support and gctest

Do not use 'ifndef AO_CLEAR' in mark, pthread_support and gctest
(code refactoring of commits 272283d5b, d6c7ee31d, 2178db2b6)

* include/private/gc_priv.h [THREADS && !NN_PLATFORM_CTR
&& !SN_TARGET_ORBIS && !SN_TARGET_PSP2 && !AO_HAVE_compiler_barrier]
(AO_HAVE_compiler_barrier): Define private macro (after include
gc_atomic_ops.h).
* mark.c [!BASE_ATOMIC_OPS_EMULATED] (GC_noop6): Call
AO_compiler_barrier() only if AO_HAVE_compiler_barrier.
* pthread_support.c [(USE_SPIN_LOCK || !NO_PTHREAD_TRYLOCK)
&& !BASE_ATOMIC_OPS_EMULATED] (GC_pause): Likewise.
* tests/test.c (AO_t): Define only if AO_HAVE_compiler_barrier is not
defined.