avr port: various changes

[Jeffrey A Law]

  In message < 200005101531.RAA18125@marekm.home >you write:
  > > Marek !
  > > I added your patches inside my changes.
  > 
  > Thanks.  I haven't seen any objections to the expmed.c tweak for
  > signed int / 2^N divide (compare BRANCH_COST with cost of one or
  > two shifts) - would it be possible to apply that too?
It needs to be reviewed.  It will be explicitly approved or rejected at which
point Denis can install or revise it as necessary.
jeff