[rfc] Add per frame frame_saved_pc()

[Andrew Cagney]

> There are currently three (er 4) frame_saved_pc() functions:

Hmm, found another one ....

> frame_saved_pc(dummy)
> frame_saved_pc(CFI)
> frame_saved_pc(``old style with frame init saved regs'')
> frame_saved_pc(NULL) === read_pc() (but only sort of)

frame_saved_pc(``sigtramp'')

Andrew