i#1551 port to ARM: TLS exit

We need some way to determine whether a thread has exited (for
deadlock_avoidance_unlock, e.g.) after dcontext and os_tls are freed.  For
now we store -1 in this slot and assume the app will never use that value
(we test that assumption by looking for our sentinel on every entrance to
DR in debug build).

i#1578 covers supporting detach on ARM where we should swap back to
os_tls->app_tls_swap instead and find some other solution here.

Review-URL: https://codereview.appspot.com/185480044