x86 backend update

[Matthew Fluet]

> I tried to do a self compile with the new native backend, and it seems
> at the very least to be a lot slower, at the worst, nonterminating. 
> It's been running for 2.5 hours, and is still sitting in the backend. 
> This is a G0 building a G1, so asserts will be turned on.  Do you think
> asserts are prohibitively slow for a self compile?  Any other ideas? 
> Here is the transcript. 

Very strange.  I did a G0 building a G1 self-compile with this version the
other day without any difficulties.  I made one or two minor modifications
(mostly just commenting out some extra debugging and printing) when I sent
out the update.  I'll start a compile of my own going and see what's going
on.