arm64: Clear td_frame when returning to userspace

When returning from an exception to userspace clear the saved td_frame.
On the next exception this should point to the frame, however this is
not guaranteed.

To ensure the trap frame pointer is either valid or NULL clear it
before returning to userspace in the EL0 synchronous exception handler.

Reviewed by: kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44807