[vm] More efficiently access thread CPU time on Mac.

TEST=ci
Bug: #47850
Change-Id: I5719c42b4e2aa0d1a1b02c5e274cdb9b93177ca1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240148
Reviewed-by: Ben Konyi <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/os_macos.cc

@@ -106,6 +106,12 @@ int64_t OS::GetCurrentMonotonicMicros() {
 }
 
 int64_t OS::GetCurrentThreadCPUMicros() {
+  if (__builtin_available(macOS 10.12, iOS 10.0, *)) {
+    // This is more efficient when available.
+    return clock_gettime_nsec_np(CLOCK_THREAD_CPUTIME_ID) /
+           kNanosecondsPerMicrosecond;
+  }
+
   mach_msg_type_number_t count = THREAD_BASIC_INFO_COUNT;
   thread_basic_info_data_t info_data;
   thread_basic_info_t info = &info_data;