Fix the Mac build on x86-64.

Change-Id: I4ed3783a96d844de0b0a295df26d0a48c02a3726
diff --git a/runtime/mem_map.cc b/runtime/mem_map.cc
index 8d987df..1074253 100644
--- a/runtime/mem_map.cc
+++ b/runtime/mem_map.cc
@@ -72,7 +72,7 @@
 
 std::multimap<void*, MemMap*> MemMap::maps_;
 
-#if defined(__LP64__) && !defined(__x86_64__)
+#if USE_ART_LOW_4G_ALLOCATOR
 // Handling mem_map in 32b address range for 64b architectures that do not support MAP_32BIT.
 
 // The regular start of memory allocations. The first 64KB is protected by SELinux.
@@ -235,7 +235,7 @@
   // A page allocator would be a useful abstraction here, as
   // 1) It is doubtful that MAP_32BIT on x86_64 is doing the right job for us
   // 2) The linear scheme, even with simple saving of the last known position, is very crude
-#if defined(__LP64__) && !defined(__x86_64__)
+#if USE_ART_LOW_4G_ALLOCATOR
   // MAP_32BIT only available on x86_64.
   void* actual = MAP_FAILED;
   if (low_4gb && expected == nullptr) {
@@ -299,7 +299,7 @@
   }
 
 #else
-#ifdef __x86_64__
+#if defined(__LP64__)
   if (low_4gb && expected == nullptr) {
     flags |= MAP_32BIT;
   }