go.net/ipv4: make use of go generate to create system adaptation files
go.net/ipv4: make use of go generate to create system adaptation files

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/160800043

dafyddcrosby/net,flowlo/go-net,mckinney-subgraph/net,yangxianjun/net,albertjin/golang-net,flowlo/go-net,muzining/net,cesanta/goxnet,cmars/tools,cmars/tools,veliamsli/net,kiip/net,gomini/net,DramaFever/golang.org-x-net,mckinney-subgraph/net,subuk/net,chenzlabs/net,muzining/net,chenzlabs/net,cmars/tools,zabawaba99/net,zabawaba99/net,cesanta/goxnet,foreversmart/net,raboof/net,cmars/tools,DramaFever/golang.org-x-net,dafyddcrosby/net,subuk/net,foreversmart/net,kiip/net,w4ngyi/net,yangxianjun/net,golang/net,w4ngyi/net,albertjin/golang-net,gomini/net,veliamsli/net,raboof/net