riscv: save the thread pointer in both modes

The contents of frame->tf_tp are uninitialized if accessed by DTrace (in
probe context), resulting in a panic when trying to access the memory
pointed to by tp. This saves the thread pointer to the trap frame when
handling both userland and kernel exceptions.

Reviewed by: markj, mhorne
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39582