arch/tile: move user_exit() to early kernel entry sequence

This ensures that we always notify context tracking that we
have exited from user space no matter how we enter the kernel.
It is similar to how arm64 handles context tracking, for example.

This allows the removal of all the exception_enter() calls that
were added in commit 49e4e156

 ("tile: support CONTEXT_TRACKING and
thus NOHZ_FULL").

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>