riscv: Panic on PMP errors

Load/store/fetch access exceptions always indicate a violation of a PMP
rule. We can't treat those as page faults, because updating the page
table and trying again will only result in exactly the same access
exception recurring. This leaves us in an endless exception loop.

We cannot recover from these exceptions, so panic instead.

Reviewed by: jhb
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D26544