Enable the four custom instructions

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 first patch of the group.

This patch is a Makefile tweak for riscv-opcodes so that the custom instructions get added to encoding.h in the following components:

* riscv-isa-sim
* riscv-pk
* riscv-test-env