Legacy floating point lifter

[percontation]

Portions of this code that were formerly ForAllSecure proprietary
are hereby released under the terms of the MIT license.

[percontation]

This is basically just the patch that @ivg formulated, but I have with proper authority edited out the notices that this is proprietary code.

I added in a couple fixes, but there's possibly still more to be made before all this is really working. (There's definitely still issues in the Mayhem code on our side that's using this FP lifter, that are blocking me from confirming that stuff largely works as intended.)

[percontation]

(Side note: Thanks for all the help Ivan! I've been working off a fork with this patch already applied, so, there's no pressure from our side to merge quickly. This PR is being issued right now because I was just informed that there was value in having all this out in public sooner rather than later.)

[ivg]

I will add tests on Monday and will merge it then. Thanks!

[percontation]

FWIW: There's some critical bug fixes to the x87 semantics that I'm working on, but, since the test added here seems to be SSE then that might work fine.

I feel like it'll be most efficient if I hold off on submitting my fixes until I've got a full "working run" of the thing I'm trying to test right now (that way I can validate that my fixes are actually an improvement 😛), but @ivg what would you prefer between:

1. I'll leave this PR as is, and send a follow up PR (sometime in the next couple of weeks) with float bugfixes.
2. I can put the the current crop of bugfixes that are ready in this PR right now.
3. I can keep adding to this PR until I'm done with my current sprint of fixes, and ping when I'm no longer actively pursing short-term issues (again, that'll be up to 2 weeks)