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

sys/riscv/riscv/exception.S

@@ -56,6 +56,8 @@ __FBSDID("$FreeBSD$");
 	/* Load our pcpu */
 	sd	tp, (TF_TP)(sp)
 	ld	tp, (TF_SIZE)(sp)
+.else
+	sd	tp, (TF_TP)(sp)
 .endif
 
 	sd	t0, (TF_T + 0 * 8)(sp)