Log Message:
Add a package for Go 1.12.

Some of the highlights include opt-in support for TLS 1.3, improved modules
support (in preparation for being the default in Go 1.13), support for
windows/arm, and improved macOS & iOS forwards compatibility.

See https://blog.golang.org/go1.12.

In pkgsrc, this is _not_ the default version for Go package builds just yet.