Use sched_yield() as fallback for preemption yield.

Signed-off-by: Kim Topley <ktopley@apple.com>

Author: triplef

src/shims/yield.h

@@ -157,8 +157,8 @@ void *_dispatch_wait_for_enqueuer(void **ptr);
 #define _dispatch_preemption_yield(n) { (void)n; Sleep(0); }
 #define _dispatch_preemption_yield_to(th, n) { (void)n; Sleep(0); }
 #else
-#define _dispatch_preemption_yield(n) { (void)n; pthread_yield(); }
-#define _dispatch_preemption_yield_to(th, n) { (void)n; pthread_yield(); }
+#define _dispatch_preemption_yield(n) { (void)n; sched_yield(); }
+#define _dispatch_preemption_yield_to(th, n) { (void)n; sched_yield(); }
 #endif // HAVE_MACH
 
 #pragma mark -