Merge commit d989ffd10 from llvm git (by Dimitry Andric):

Implement computeHostNumHardwareThreads() for FreeBSD

  This retrieves CPU affinity via FreeBSD's cpuset(2) API, and makes
  LLVM respect affinity settings configured by the user via the
  cpuset(1) command.

  In particular, this allows to reduce the number of threads used on
  machines with high core counts, which can interact badly with
  parallelized build systems. This is particularly noticable with lld,
  which spawns lots of threads even for linking e.g. hello_world!

  This fix is related to PR48193, but does not adress the more
  fundamental problem, which is that LLVM by default grabs as many CPUs
  and/or threads as possible.

  Reviewed By: MaskRay

  Differential Revision: https://reviews.llvm.org/D92271

Originally by:	mjg
MFC after:	1 week

contrib/llvm-project/llvm/lib/Support/Unix/Threading.inc

@@ -28,6 +28,7 @@
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
 #include <errno.h>
+#include <sys/cpuset.h>
 #include <sys/sysctl.h>
 #include <sys/user.h>
 #include <unistd.h>
@@ -282,7 +283,13 @@ SetThreadPriorityResult llvm::set_thread_priority(ThreadPriority Priority) {
 #include <thread>
 
 int computeHostNumHardwareThreads() {
-#ifdef __linux__
+#if defined(__FreeBSD__)
+  cpuset_t mask;
+  CPU_ZERO(&mask);
+  if (cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_TID, -1, sizeof(mask),
+                         &mask) == 0)
+    return CPU_COUNT(&mask);
+#elif defined(__linux__)
   cpu_set_t Set;
   if (sched_getaffinity(0, sizeof(Set), &Set) == 0)
     return CPU_COUNT(&Set);