Fix the build for N3DS

Fix the build for N3DS
(part of commit 0f9ef2d from Unity-Technologies/bdwgc)

Issue #173 (bdwgc).

* include/gc.h [GC_BUILD && NN_PLATFORM_CTR] (GC_on_thread_event_proc):
Define type.
* include/private/gc_locks.h [NN_PLATFORM_CTR || NINTENDO_SWITCH]
(GC_lock, GC_unlock): Declare function.
* include/private/gc_locks.h [NN_PLATFORM_CTR || NINTENDO_SWITCH]
(UNCOND_LOCK, UNCOND_UNLOCK): Define macro.
* include/private/gc_priv.h [THREADS && !SN_TARGET_ORBIS
&& !SN_TARGET_PSP2]: Do not include gc_atomic_ops.h if NN_PLATFORM_CTR.
* pthread_support.c [USE_SPIN_LOCK || !NO_PTHREAD_TRYLOCK] (GC_pause):
Call GC_noop1 instead of AO_compiler_barrier unless AO_CLEAR defined.