Improved undefined-function backtrace on non-x86oids.

Improved undefined-function backtrace on non-x86oids.

  * Instead of "bogus stack frame", present undefined function
frames as "undefined function".

  * Implement this by checking specifically for the undefined
function trampoline function object (which otherwise fails
pointer validation) in MAKE-LISP-OBJ, and allowing the existing
logic to do what it was always supposed to do.

  * Tested on PPC only, but should work on the other affected
targets.