Don't call computeHostNumPhysicalCores when LLVM_ENABLE_THREADS is off

[akhuang]

Summary

Fix change from 8404aeb56a73 to avoid calling
computeHostNumPhysicalCores if LLVM_ENABLE_THREADS is off.

[rnk]

lgtm

[rnk]

llvm/lib/Support/Host.cpp

1329

As we discovered later, this should be spelled LLVM_ENABLE_THREADS != 0, and something needs to be done to fix SupportTests

[akhuang]

Fix test and macro spelling

[rnk]

lgtm

To be clear, this landed as rGcb36bfa3de3ecba0e30de766999243eb72360533.

llvm/lib/Support/Host.cpp

if (count < 1) {
     nm[0] = CTL_HW;
     nm[1] = HW_AVAILCPU;
     sysctl(nm, 2, &count, &len, NULL, 0);
     if (count < 1)
       return -1;
   }
   return count;
 }
-#elif defined(_WIN32)
+#elif defined(_WIN32) && LLVM_THREADS_ENABLED
 // Defined in llvm/lib/Support/Windows/Threading.inc
 int computeHostNumPhysicalCores();
 #else
 // On other systems, return -1 to indicate unknown.
 static int computeHostNumPhysicalCores() { return -1; }
 #endif
 
 int sys::getHostNumPhysicalCores() {