ld: change DWARF output for structs
The debug/dwarf package cannot parse the format generated here,
but the format can be changed so it does.
After this edit, tweaking the expression defining the offset
of a struct field, the dwarf package can parse the tables (again?).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/105710043
liblink: fix warnings on Plan 9

warning: /usr/go/src/liblink/asm5.c:720 set and not used: m
warning: /usr/go/src/liblink/asm5.c:807 set and not used: c

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/108570043
cmd/ld: add go-specific dwarf type information

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/116720043
make.bat: return exit code to dashboard builder

Fixes issue 7806.

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://codereview.appspot.com/104690043