cmd/6c, cmd/8c: fix print format for Prog

The FmtLong flag should only be used with the %D verb
when printing an ATEXT Prog. It was erroneously used
for every Prog except ADATA. This caused a preponderance
of exclamation points, "!!", in the assembly listings.

I also cleaned up the code so that the list.c files look
very similar. Now the real differences are easily spotted
with a simple diff.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7128045