This comes out of a discussion with Daniel, about how gcc
does not generate the right dwarf info to allow correct
frame unwinding in function epilogues, causing frame_unwind
to return bad results.

It's necessary for reverse-step, which will frequently step
backward to the return instruction of a function.  But it also
provides an improvement for forward debugging, in that now,
without this change, if you STEPI until you are at the return
instruction, you will get a bad backtrace.

The infrun changes that take advantage of this patch will follow
separately.

Michael