riscv: Handle access faults in user mode

[kp]

Summary

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.

Sponsored by: Axiado
Suggested by: jrtc27