Add os_unfair_lock support.

OS X 10.12 deprecated OSSpinLock; os_unfair_lock is the recommended
replacement.

include/jemalloc/internal/jemalloc_internal_decls.h

@@ -17,6 +17,9 @@
 #    include <sys/uio.h>
 #  endif
 #  include <pthread.h>
+#  ifdef JEMALLOC_OS_UNFAIR_LOCK
+#    include <os/lock.h>
+#  endif
 #  ifdef JEMALLOC_GLIBC_MALLOC_HOOK
 #    include <sched.h>
 #  endif

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -60,6 +60,11 @@
  */
 #undef JEMALLOC_HAVE_MADVISE
 
+/*
+ * Defined if os_unfair_lock_*() functions are available, as provided by Darwin.
+ */
+#undef JEMALLOC_OS_UNFAIR_LOCK
+
 /*
  * Defined if OSSpin*() functions are available, as provided by Darwin, and
  * documented in the spinlock(3) manual page.

include/jemalloc/internal/mutex.h

@@ -5,6 +5,9 @@ typedef struct malloc_mutex_s malloc_mutex_t;
 
 #ifdef _WIN32
 #  define MALLOC_MUTEX_INITIALIZER
+#elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
+#  define MALLOC_MUTEX_INITIALIZER					\
+     {OS_UNFAIR_LOCK_INIT, WITNESS_INITIALIZER(WITNESS_RANK_OMIT)}
 #elif (defined(JEMALLOC_OSSPIN))
 #  define MALLOC_MUTEX_INITIALIZER {0, WITNESS_INITIALIZER(WITNESS_RANK_OMIT)}
 #elif (defined(JEMALLOC_MUTEX_INIT_CB))
@@ -35,6 +38,8 @@ struct malloc_mutex_s {
 #  else
 	CRITICAL_SECTION	lock;
 #  endif
+#elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
+	os_unfair_lock		lock;
 #elif (defined(JEMALLOC_OSSPIN))
 	OSSpinLock		lock;
 #elif (defined(JEMALLOC_MUTEX_INIT_CB))
@@ -88,6 +93,8 @@ malloc_mutex_lock(tsdn_t *tsdn, malloc_mutex_t *mutex)
 #  else
 		EnterCriticalSection(&mutex->lock);
 #  endif
+#elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
+		os_unfair_lock_lock(&mutex->lock);
 #elif (defined(JEMALLOC_OSSPIN))
 		OSSpinLockLock(&mutex->lock);
 #else
@@ -109,6 +116,8 @@ malloc_mutex_unlock(tsdn_t *tsdn, malloc_mutex_t *mutex)
 #  else
 		LeaveCriticalSection(&mutex->lock);
 #  endif
+#elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
+		os_unfair_lock_unlock(&mutex->lock);
 #elif (defined(JEMALLOC_OSSPIN))
 		OSSpinLockUnlock(&mutex->lock);
 #else