RFC: %ebp-based backtrace patch

[Mark Kettenis]

> Date: Tue, 7 Jul 2009 09:00:40 -0400
> From: Daniel Jacobowitz <drow@false.org>
> 
> On Mon, Jul 06, 2009 at 11:57:29PM +0200, Mark Kettenis wrote:
> > Makes sense to me.  If we have no clue where we are anymore, taking
> > the gamble that %ebp is a valid frame pointer probably has better odds
> > than that %esp points to a valid frame.  This will need a comment
> > though.  I'll take care of that.
> 
> Thank you.  Would you like me to check in the code, or will you do
> both together?

Before comitting this diff, I ran the testsuite, and noticed a
regression.  The problem is that if you do a function call through a
null-function-pointer or an otherwise corrupt function pointer, and
get a SIGSEGV, the backtrace no longer shows the frame that did the
function call.

Until we come up with a way to fix this issue, I'm not going to commit
the diff.