Implementation of FENCE instruction

Created by: hyf6661669

I find that FENCE instruction in Ariane will invalidate every set of dcache and write back the cacheline which is dirty. In https://github.com/riscv/riscv-isa-manual/issues/252, it is said that flushing the whole dcache is not necessary. I also think that flushing the whole dcache is too costly. Do you have any suggestion about a more effective implementation for FENCE isntruction?