This patch should apply cleanly to the current head of hexl.el, I've 
included a copy of hexl.el with the patch applied.

It allows hexl.el to take advantage of the fact that the hexl binary can
emit hexlified data with varying word sizes.

I originally did this work in emacs21 for an arm linux kernel hacker
I know, I guess it might be useful to others.

It didn't get into emacs22 because emacs22 was about to go out and new 
features were not being accepted.

hexl.el doesn't appear to have changed much between 22 and 23, so hopefully
it can be considered for inclusion this time around.