fix a bug of the android Makefile for nsync library

NDK doesn't support thread_local variables which require destructors,
so on Android it should use __thread instead.

Observations:
* ProtoBuf and other libraries are not using thread_local on Android.
* In Tensorflow, there's a "thread_local" in code about CUDA,
  which should be safe enough.

More discussions are on https://github.com/android-ndk/ndk/issues/360 .