atomics: Favor compiler primitives over macOS APIs. (#676)

In macOS 10.12, the `OSMemoryBarrier` and related APIs were deprecated
in favor of using `std::atomic`. On the way to supporting `std::atomic`,
we can favor using the "Intel primitives" which are also available on
macOS.

src/hb-atomic-private.hh

@@ -70,6 +70,29 @@ typedef LONG hb_atomic_int_impl_t;
 #define hb_atomic_ptr_impl_cmpexch(P,O,N)	(InterlockedCompareExchangePointer ((void **) (P), (void *) (N), (void *) (O)) == (void *) (O))
 
 
+#elif !defined(HB_NO_MT) && defined(HAVE_INTEL_ATOMIC_PRIMITIVES)
+
+typedef int hb_atomic_int_impl_t;
+#define HB_ATOMIC_INT_IMPL_INIT(V) (V)
+#define hb_atomic_int_impl_add(AI, V)		__sync_fetch_and_add (&(AI), (V))
+
+#define hb_atomic_ptr_impl_get(P)		(void *) (__sync_synchronize (), *(P))
+#define hb_atomic_ptr_impl_cmpexch(P,O,N)	__sync_bool_compare_and_swap ((P), (O), (N))
+
+
+#elif !defined(HB_NO_MT) && defined(HAVE_SOLARIS_ATOMIC_OPS)
+
+#include <atomic.h>
+#include <mbarrier.h>
+
+typedef unsigned int hb_atomic_int_impl_t;
+#define HB_ATOMIC_INT_IMPL_INIT(V) (V)
+#define hb_atomic_int_impl_add(AI, V)		( ({__machine_rw_barrier ();}), atomic_add_int_nv (&(AI), (V)) - (V))
+
+#define hb_atomic_ptr_impl_get(P)		( ({__machine_rw_barrier ();}), (void *) *(P))
+#define hb_atomic_ptr_impl_cmpexch(P,O,N)	( ({__machine_rw_barrier ();}), atomic_cas_ptr ((void **) (P), (void *) (O), (void *) (N)) == (void *) (O) ? true : false)
+
+
 #elif !defined(HB_NO_MT) && defined(__APPLE__)
 
 #include <libkern/OSAtomic.h>
@@ -96,29 +119,6 @@ typedef int32_t hb_atomic_int_impl_t;
 #endif
 
 
-#elif !defined(HB_NO_MT) && defined(HAVE_INTEL_ATOMIC_PRIMITIVES)
-
-typedef int hb_atomic_int_impl_t;
-#define HB_ATOMIC_INT_IMPL_INIT(V) (V)
-#define hb_atomic_int_impl_add(AI, V)		__sync_fetch_and_add (&(AI), (V))
-
-#define hb_atomic_ptr_impl_get(P)		(void *) (__sync_synchronize (), *(P))
-#define hb_atomic_ptr_impl_cmpexch(P,O,N)	__sync_bool_compare_and_swap ((P), (O), (N))
-
-
-#elif !defined(HB_NO_MT) && defined(HAVE_SOLARIS_ATOMIC_OPS)
-
-#include <atomic.h>
-#include <mbarrier.h>
-
-typedef unsigned int hb_atomic_int_impl_t;
-#define HB_ATOMIC_INT_IMPL_INIT(V) (V)
-#define hb_atomic_int_impl_add(AI, V)		( ({__machine_rw_barrier ();}), atomic_add_int_nv (&(AI), (V)) - (V))
-
-#define hb_atomic_ptr_impl_get(P)		( ({__machine_rw_barrier ();}), (void *) *(P))
-#define hb_atomic_ptr_impl_cmpexch(P,O,N)	( ({__machine_rw_barrier ();}), atomic_cas_ptr ((void **) (P), (void *) (O), (void *) (N)) == (void *) (O) ? true : false)
-
-
 #elif !defined(HB_NO_MT) && defined(_AIX) && defined(__IBMCPP__)
 
 #include <builtins.h>