avr and frame unwinding

[Andrew Cagney]

Can you try the exact same operation with a GDB that doesn't have the 
changes?  That way it should be possible to compare the two traces 
side-by-side and see where things are going wrong.

A guess is that it is getting the unwound PC value wrong.  For the d10v 
I had to make two tweaks:

- abort the prologue scanner when it reached PC if that is before the 
end of the prologue
This stopped the prologue getting the unwound PC's location wrong.  It 
may not have yet executed the save PC instruction.

- track the register that the PC is in before it is saved
The code needs to be able to unwind the PC value in the prologue before 
it has been saved on the stack.

Andrew