riscv vmm: implement SBI RFNC extension.

The RISC-V SBI (Supervisor Binary Interface) RFNC (fence) extension is used
to provide a standardized mechanism for enforcing memory and I/O ordering
between different execution contexts or cores in a system. Specifically, it
allows supervisors (such as an operating system kernel) to request certain
synchronization operations across CPUs or harts (hardware threads) via the
SBI.

Differential Revision: https://reviews.freebsd.org/D48441