Restore dlmalloc mmap threshold to 64k

Restoring DEFAULT_MMAP_THRESHOLD to 64k, the way it was before
999089181ef60bb67e1a49f2cf6f4ec608a7caf8.

This forces allocations in the 64k-256k range to be mmaped.

Change-Id: Iace55ed638edd272b3e94fa6cd2ddd349042be84
Signed-off-by: Rom Lemarchand <romlem@google.com>