Store the original value of stack pointer to the exception frame

(the value we had before supervisor exception occurred).
This helps consumers (e.g. DTrace) to not proceed additional calculations.

Sponsored by:	DARPA, AFRL
Sponsored by:	HEIF5

sys/riscv/riscv/exception.S

@@ -93,7 +93,9 @@ __FBSDID("$FreeBSD$");
 
 .if \el == 1
 	/* Store kernel sp */
-	sd	sp, (TF_SP)(sp)
+	li	t1, TF_SIZE
+	add	t0, sp, t1
+	sd	t0, (TF_SP)(sp)
 .else
 	/* Store user sp */
 	csrr	t0, sscratch