godoc: forward /pkg/C links to /cmd/cgo. Fixes issue 5651.
godoc: forward /pkg/C links to /cmd/cgo.
Fixes issue 5651.

R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/58660043

d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,8limbs/gotools