Fix a portable build breakage

  http://android-build/builds/git_dalvik-dev-linux-mysid-userdebug_portable/848398/logs/build.log

 due to

  https://googleplex-android.googlesource.com/platform/art/+/3b4c18933c24b8a33f38573c2ebcdb9aa16efeb5

Change-Id: I89ef5f77dfc640efbe3378549e22bb699da3149f
diff --git a/runtime/gc/allocator/dlmalloc.h b/runtime/gc/allocator/dlmalloc.h
index 07ebd1c..19159b1 100644
--- a/runtime/gc/allocator/dlmalloc.h
+++ b/runtime/gc/allocator/dlmalloc.h
@@ -18,6 +18,8 @@
 #define ART_RUNTIME_GC_ALLOCATOR_DLMALLOC_H_
 
 // Configure dlmalloc for mspaces.
+// Avoid a collision with one used in llvm.
+#undef HAVE_MMAP
 #define HAVE_MMAP 0
 #define HAVE_MREMAP 0
 #define HAVE_MORECORE 1