Add instruction-based regression tests about simeng HOT 1 CLOSED

Ideally, the regression tests would be written (and stored) as plain-text assembly files (.s), which would make them easy to view, edit, and track in git. This does however mean that we'd need access to an assembler at build time in order to produce the test binaries that SimEng can consume.

One option for this would be to use LLVM. This is a somewhat heavyweight dependency, but I'm raising it as an option because there's also other functionality it provides that would likely be useful in the future (e.g. APIs for handling multiple binary formats, the TableGen interface, and maybe the stuff that describes port configuration and instruction latencies for different architectures). It's also possible to build just a subset of LLVM to get the specific tools or libraries that are required. I tested building just llvm-mc and it "only" took a couple of minutes on my laptop.

Thoughts?

from simeng.