Improved undefined-function backtrace on x86oids.

Improved undefined-function backtrace on x86oids.

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

  * Implement this by checking specifically for escaped frames
with the program counter within the range between the start of
undefined_tramp and the start of whichever function follows it
in $ARCH-assem.S.