[rfc] Add per frame frame_saved_pc()

[Andrew Cagney]

Hello,

Back to (trumpet noises) CFI and more fall-out from the MIPS and generic 
dummy frames.

- The CFI doco describes a ``return_address_register'' which ``[...] 
might not correspond to a machine register''.  This means a CFI specific 
frame saved PC method.

- Given a generic dummy frame, it knows exactly where the callers frame 
PC is.  No need to ask the target code.

With this in mind, I'd like to propose making frame_saved_pc() a 
per-frame method.

Thoughts?

To be honest, it is largely motivated by me having to wade though the 
MIPS code and add if(PC_IN_CALL_DUMMY()) to everything - I'm hopeing to 
avoid it next time :-)

After this is comes FRAME_CHAIN() but I think that needs more careful 
consideration.

Andrew