riscv: Handle supervisor instruction page faults

We should never take instruction page faults when in the kernel, but by
using the standard page fault code we should get a more-informative
message about faulting on a NOFAULT page rather than branching to the
default case here and printing an "Unknown kernel exception ..."
message.

Reviewed by: jhb (mentor), markj
Approved by: jhb (mentor), markj
Differential Revision: https://reviews.freebsd.org/D26685