8g/cgen.c:
8g/gobj.c
. dropped unnecessary assignments;
8g/gg.h
. added varargckk pragmas;
8g/ggen.c
. dropped duplicate assignment;
8g/gsubr.c
. adjusted format in print statement;
. dropped unnecessary assignment;
. replaced GCC's _builtin_return_address(0) with Plan 9's
  getcallerpc(&n) which is defined as a macro in <u.h>;
8g/list.c
. adjusted format in snprint statement;
8g/opt.h
. added varargck pragma (Adr*) that is specific for the invoking
  modules;
8g/peep.c
. dropped unnecessary incrementation;

R=rsc
CC=golang-dev
https://golang.org/cl/4974044