i#1551 port to ARM: TLS init for early injection

The app has not set up its TLS, leaving DR with no slot for swapping.
Strangely, tpidrro is not zero, so we add an explicit tls_early_init()
which calls SYS_set_tls on a static buffer to serve us until the app sets
up its TLS.

Review-URL: https://codereview.appspot.com/190200043