net: only "build" empty.s in non-cgo mode
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

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go