Add support for clock_gettime_nsec_np()

Prefer clock_gettime_nsec_np(CLOCK_UPTIME_RAW) to mach_absolute_time().

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -117,6 +117,11 @@
  */
 #undef JEMALLOC_HAVE_CLOCK_REALTIME
 
+/*
+ * Defined if clock_gettime_nsec_np(CLOCK_UPTIME_RAW) is available.
+ */
+#undef JEMALLOC_HAVE_CLOCK_GETTIME_NSEC_NP
+
 /*
  * Defined if _malloc_thread_cleanup() exists.  At least in the case of
  * FreeBSD, pthread_key_create() allocates, which if used during malloc