cmd/6g, cmd/gc, cmd/ld: fix Plan 9 amd64 warnings
warning: src/cmd/6g/reg.c:671 format mismatch d VLONG, arg 4
warning: src/cmd/gc/pgen.c:230 set and not used: oldstksize
warning: src/cmd/gc/plive.c:877 format mismatch lx UVLONG, arg 2
warning: src/cmd/gc/walk.c:2878 set and not used: cbv
warning: src/cmd/gc/walk.c:2885 set and not used: hbv
warning: src/cmd/ld/data.c:198 format mismatch s IND FUNC(IND CHAR) INT, arg 2
warning: src/cmd/ld/data.c:230 format mismatch s IND FUNC(IND CHAR) INT, arg 2
warning: src/cmd/ld/dwarf.c:1517 set and not used: pc
warning: src/cmd/ld/elf.c:1507 format mismatch d VLONG, arg 2
warning: src/cmd/ld/ldmacho.c:509 set and not used: dsymtab
R=golang-dev, gobot, rsc
CC=golang-dev
https://golang.org/cl/
36740045