Enable support for the four custom instructions

[arunthomas]

I'd like to experiment with custom instructions in riscv-isa-sim, so I have a few patches to enable the four custom instructions. This is the fourth and final patch of the group.

This patch updates the generated encoding.h (generated from riscv-opcodes).

[arunthomas]

Here's the full list of related pull requests:

ucb-bar/riscv-opcodes#1 ucb-bar/riscv-isa-sim#4 ucb-bar/riscv-pk#3 ucb-bar/riscv-test-env#1

[yunsup]

Thanks.