[rfa] Attach vsyscall support for GNU/Linux

[Andrew Cagney]

>>But there's another reason why I'm against this patch.  It changes the
>>order of the unwinders for all the other i386 targets too.  Please
>>find a way to do this such that it affect Linux only.
>>
>>Sorry, I didn't realize this before...

I'd not get too hung up on this one.  The current frame-unwind code 
sniff heuristic came about because it ``worked for me''.   We're going 
to have to eventually replace it with something more complicated and 
until then, what ever.

Sniffing a signal frame definitly takes higher priority than sniffing a 
normal frame.

> OK, if the rest of the patch reaches consensus I will try to figure out
> how to do this.  It will probably just require conditional action based
> on the osabi enum.  Not hard.

Andrew