Here's an updated version of a little hack I've been using since GDB 6.0.
If we are in a nested normal frame, i.e. something whose next frame is a
function that it called in the normal way, and we didn't find a saved PC,
we're going to be stuck in a loop. We might have been able to figure out
the frame size, but not where the return address was stored; as the comment
says, this happens in glibc's clone function. Of course the problem there
is that it _doesn't_ save $ra in the normal fashion; it won't return.
Without this patch schedlock.exp falls apart, because backtraces continue
forever printing "clone()" on every line.
OK?