Remove macOS 32bit support

Overview

With Go 1.15 that will be out on August 2020, it will remove support for 32bit macos support darwin/386. This was posted in https://golang.org/doc/go1.14#darwin and was merged in for 1.15 https://go-review.googlesource.com/c/go/+/226985/. Since we want to try and stay up to date to the latest version of Go we should do this "breaking change" on a major release.

Proposal

Stop compiling darwin/386 in %13.0