runtime: more NOPTR

Fixes linux builds (_vdso); may fix others.
I can at least cross-compile cmd/go for every
implemented system now.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/142630043
net: only "build" empty.s in non-cgo mode

In cgo mode it gets passed to gcc, and on ARM
it appears that gcc does not support // comments.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/142640043