cmd/go: a package that uses cgo does not compile c code in parallel

[xiaoyulei]

I have a package that uses cgo and some C files in it. When I compile, it will compile c files in sequence, not parallel. This will take most compile time. I think we should compile it in parallel automatically or add some parameter to decide by user.

[davecheney]

If you move the cgo code into a separate package, then go tool will be able to compile it once and cache the result.

[ianlancetaylor]

Dup of #9887.