Improve the reporting of the number of CPUs on Mac OS

runtime/bin/platform_macos.cc

Index: runtime/bin/platform_macos.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_macos.cc
index 0e7f2f8f6ea3b640e0094719e001cab4d1e3647e..6c83fc81b0be67333b94541fced18e36a4bbd719 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_macos.cc
@@ -6,6 +6,8 @@
 #if defined(TARGET_OS_MACOS)
 
 #include <mach-o/dyld.h>
+#include <sys/types.h>
+#include <sys/sysctl.h>
 
 #include "bin/file.h"
 #include "bin/platform.h"
@@ -37,7 +39,14 @@ bool Platform::Initialize() {
 
 
 int Platform::NumberOfProcessors() {
-  return sysconf(_SC_NPROCESSORS_ONLN);
+  int32_t cpus = -1;
+  size_t cpus_length = sizeof(cpus);
+  if (sysctlbyname("hw.logicalcpu", &cpus, &cpus_length, NULL, 0) == 0) {
+    return cpus;
+  } else {
+    // Failed, fallback to using sysconf.
+    return sysconf(_SC_NPROCESSORS_ONLN);
+  }
 }