dos: change the link procedure to use negative pointers below DGROUP

dos: change the link procedure to use negative pointers below DGROUP

Use negative pointer values below DGROUP, i.e. for the .payload
segment.  This makes it a lot easier to figure out if we are dealing
with pointers that shouldn't come from where they do, since the DGROUP
and .payload segments no longer overlap, and in fact are congruent.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>