runtime: fix netbsd, windows build

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/7539043
net/http/httputil: fix build

TBR=bradfitz
CC=golang-dev
https://codereview.appspot.com/7540043
cmd/cgo: split cgo_export into cgo_export_static and cgo_export_dynamic

Also emit cgo_ldflag pragmas.

R=golang-dev, remyoudompheng, iant
CC=golang-dev
https://codereview.appspot.com/7530043