Remove stackadjust and stackoffset() from ncg.

Remove stackadjust and stackoffset() from ncg.

This feature has never been used since its introduction, more than 3
years ago, in David Given's commit c93cb69 of May 8, 2013.  The commit
was for "PowerPC and M68K work".  I am not undoing the entire commit.
I am only removing the stackadjust and stackoffset() feature.

This commit removes the feature from my branch kernigh-linuxppc.  This
removal includes the mach/proto/ncg parts.  The default branch already
removed most of the feature, but kept the mach/proto/ncg parts.  That
removal happened in commit 81778b6 of May 13, 2013 (which was a merge;
git diff af0dede 81778b6).  The branch dtrg-experimental-powerpc
merged the default branch but without the removal.  That merge was
commit 4703db0f of Sep 15, 2016 (git diff 8c94b13 4703db0).  My branch
kernigh-linuxppc is off branch dtrg-experimental-powerpc, so I can no
longer get the removal by merging default.

David Given described the stackadjust feature in
  https://sourceforge.net/p/tack/mailman/message/30814691/

The instruction stackadjust would add a value to the offset, and the
function stackoffset() would return this offset.  One would use this
to track sp - fp, then omit the frame pointer by not keeping fp in a
register.