Improved undefined-function backtrace on PPC.

Improved undefined-function backtrace on PPC.

When there is an existing stack frame (either for a large number
of arguments or for a tail-call), undefined_tramp triggers a
strange case in build_fake_control_stack_frames() that can leave
the OCFP and LRA slots uninitialized, causing a truncated
backtrace.

  * Alter undefined_tramp to always allocate and initialize a
stack frame.

  * While we're here, tighten up the reg_CODE initialization
sequence.