darwin: invoke mach_timebase_info only once

According to @aktau, the call to `mach_timebase_info` costs 90% of the
total execution time of `uv_hrtime()`. The result of the call is static
on all existing platforms, so there is no need in invoking it multiple
times.

Signed-off-by: Fedor Indutny <fedor@indutny.com>

src/unix/darwin.c

@@ -179,12 +179,14 @@ void uv__cf_loop_signal(uv_loop_t* loop, cf_loop_signal_cb cb, void* arg) {
 
 
 uint64_t uv__hrtime(void) {
-    mach_timebase_info_data_t info;
+  static mach_timebase_info_data_t info;
 
-    if (mach_timebase_info(&info) != KERN_SUCCESS)
-      abort();
+  if ((ACCESS_ONCE(uint32_t, info.numer) == 0 ||
+       ACCESS_ONCE(uint32_t, info.denom) == 0) &&
+      mach_timebase_info(&info) != KERN_SUCCESS)
+    abort();
 
-    return mach_absolute_time() * info.numer / info.denom;
+  return mach_absolute_time() * info.numer / info.denom;
 }