This reverts commit fe574261.

This reverts commit 70b52dd5.

Refactoring support for custom instructions.

Conflicts:
	Makefile

Not every instruction in the main opcodes file is implemented by Hwacha;
at present, updating opcodes_hwacha_ut.h requires manual culling of the
unneeded instructions to avoid breaking the spike build.

Will update encoding.h in the following components:

* riscv-isa-sim
* riscv-pk
* riscv-test-env