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>
[MIPS] __dso_handle.S and __dso_handle_so.S not needed.

Global hidden variable __dso_handle is now declared in:
       bionic/libc/private/__dso_handle.h

Change-Id: I8e951a8d7c65877bafc1be23a7fff6d44d3a2846
Signed-off-by: Pete Delaney  <piet.delaney@imgtec.com>
Signed-off-by: Chao-Ying Fu  <chao-ying.fu@imgtec.com>