build: use thread_local instead of __thread for UWP build

- Causes the UWP build condition to use thread_local when
  registering threads with pj_thread_register(), as __thread is
  not available. Note: __declspec(thread) is also equivalent.

Change-Id: Ibee418d4c7084264acb0591015cdb8f72abc50ba
Tuleap: #790