[ref: cagney-unwind-20030108-branch]

This patch add the two files frame-unwind.[hc] that implement a simple 
frame unwind registrary.

Given a function that contains the address PC, the method 
frame_unwind_find_by_pc(ARCH,PC) returns methods that can unwind the 
specified function's stack frame.

At present the code knows about two frames:

dummy-frame
these are the frames that GDB creates when doing an inferior function call.

trad[itional]-frame
GDB's traditional / original prologe analysis, extra frame info, frame 
saved regs, frame.

Architectures can add to this list.  On the above branch, the d10v adds 
a rewritten d10v specific frame unwinder.  An obvious addition would be 
a CFI frame unwinder.

I'll look to commit this in a few days.
Andrew