build: fix elf builds

Corrections due to new strict type rules for data+bss.
Also disable misc/cgo/cdefstest since you can't compile C code anymore.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/148050044
cmd/go: strip -fsanitize= flags when building cgo object
Fixes issue 8788.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/142470043
runtime: more NOPTR

Fixes (or makes better) various builds.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/146280043