[RFC] Improve amd64 prologue analysis

[Joel Brobecker]

> > Now I suspect that 64-bit Windows, given its utterly retarded IL32P64
> > model, doesn't follow that ABI.  Therefore my suggestion would be to
> > only use this code on Windows.
> 
> On windows, there is some unwinding info that's generated as well.
> We just don't read it, yet. It's something we'd love to do at AdaCore
> eventually, but we're lacking the time at the moment.

One last bit of information - which I got second hand, but hopefully
accurate. I think that the GCC team is on its way to generating the
unwinding info in that format, rather than the usual DWARF-based
eh_frame/debug_frame. So the Microsoft format should become the default
even for code generated by GCC... So eventually, I think we are going
to need to add an unwinder for that, even for GCC code.

-- 
Joel