folly: fix AtomicUnorderedMap compilation on macOS

Summary:
MAP_POPULATE is not defined on this system.  Instead we will `madvise` the kernel
that we will need it so that it will populate the mapping.

Reviewed By: yfeldblum

Differential Revision: D3584325

fbshipit-source-id: ece52f3d55c475bcd41367f4e9744d6f41001cd5

folly/detail/AtomicUnorderedMapUtils.h

@@ -28,12 +28,20 @@ class MMapAlloc {
         nullptr,
         len,
         PROT_READ | PROT_WRITE,
-         MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
+        MAP_PRIVATE | MAP_ANONYMOUS
+#ifdef MAP_POPULATE
+            |
+            MAP_POPULATE
+#endif
+        ,
         -1,
         0));
     if (mem == reinterpret_cast<void*>(-1)) {
       throw std::system_error(errno, std::system_category());
     }
+#if !defined(MAP_POPULATE) && defined(MADV_WILLNEED)
+    madvise(mem, size, MADV_WILLNEED);
+#endif
 
     return mem;
   }