riscv: handle access faults in user mode

Access faults in user mode are treated like TLB misses, which leads to an
endless loop of faults. It's less serious than the same fault in kernel mode,
because we can just terminate the process, but that's not ideal.

Treat user mode access faults as a bus error.

Suggested by: jrtc27
Reviewed by: br, jhb
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D26621