The mono_thread_manage API expects to be called in GC Unsafe mode.

This has been the case for a while, but for some reason it only
caused problems in the Mono merge I'm working on now.

We don't need a matching exit call because we're shutting down
the runtime anyway, and we can't change GC modes after the runtime
shuts down.
Signed-off-by: Vincent Povirk <vincent@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>