Fix typo in NEWS

Reported-by: Douglas Katzman
Fix sigtrap_handler on ARM.

The previous change removed the undefined instruction handling,
breaking many things.
Store unibyte mapping tables more densely by specializing the vectors.

Incidentally, get the cross-compiler to dump SIGNED-BYTE arrays.
The long-awaited backquote patch.

This makes the pprinter for backquote faster, safer, and less consy,
and fixes lp#1063414.